Thread-safety panic

Getting a panic when using the package in [parallel tests](https://pkg.go.dev/testing#T.Parallel):

```golang
...
schemaCache := kubeval.NewSchemaCache()
config := kubeval.NewDefaultConfig()
config.Strict = true
config.IgnoreMissingSchemas = true
config.Quiet = true
kubeval.ValidateWithCache(someYaml, schemaCache, config)
...
```
and it works just fine when running without parallelism.
### Setup:
`go version go1.16.5 darwin/amd64`
`github.com/instrumenta/kubeval v0.16.1`

### Trace:
```
fatal error: concurrent map writes

goroutine 55 [running]:
runtime.throw(0x5681072, 0x15)
/usr/local/Cellar/go/1.16.5/libexec/src/runtime/panic.go:1117 +0x72 fp=0xc000d67368 sp=0xc000d67338 pc=0x4039db2
runtime.mapassign_faststr(0x5455740, 0xc00035e960, 0x566a8da, 0x5, 0x0)
/usr/local/Cellar/go/1.16.5/libexec/src/runtime/map_faststr.go:211 +0x3f1 fp=0xc000d673d0 sp=0xc000d67368 pc=0x40165b1
github.com/xeipuuv/gojsonschema.(*FormatCheckerChain).Add(...)
/Users/sam/go/pkg/mod/github.com/xeipuuv/gojsonschema@v0.0.0-20180816142147-da425ebb7609/format_checkers.go:99
github.com/instrumenta/kubeval/kubeval.validateAgainstSchema(0x5456040, 0xc0011707e0, 0xc000d67508, 0xc000cd9410, 0xc000e8e210, 0x0, 0x0, 0x0, 0x0, 0x0)
/Users/sam/go/pkg/mod/github.com/instrumenta/kubeval@v0.16.1/kubeval/kubeval.go:176 +0xec fp=0xc000d67430 sp=0xc000d673d0 pc=0x484b96c
github.com/instrumenta/kubeval/kubeval.validateResource(0xc00080c700, 0x100, 0x100, 0xc000cd9410, 0xc000e8e210, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
/Users/sam/go/pkg/mod/github.com/instrumenta/kubeval@v0.16.1/kubeval/kubeval.go:159 +0x2f7 fp=0xc000d67588 sp=0xc000d67430 pc=0x484af57
github.com/instrumenta/kubeval/kubeval.ValidateWithCache(0xc00080c700, 0x100, 0x100, 0xc000cd9410, 0xc000d67e78, 0x1, 0x1, 0x0, 0x0, 0x0, ...)
/Users/sam/go/pkg/mod/github.com/instrumenta/kubeval@v0.16.1/kubeval/kubeval.go:331 +0x99a fp=0xc000d67d78 sp=0xc000d67588 pc=0x484cdda
myproject.com/cmd/parallel_test.TestValidSchema.func1(0xc000602f00, 0xc000666078)
/Users/sam/myproject/cmd/another_test.go:176 +0x269 fp=0xc000d67f50 sp=0xc000d67d78 pc=0x53096e9
myproject.com/pkg/testing.runTestCase.func1(0xc000602f00)
/Users/sam/myproject/pkg/testing/testcase.go:125 +0x42 fp=0xc000d67f80 sp=0xc000d67f50 pc=0x5305c82
testing.tRunner(0xc000602f00, 0xc0016cffa0)
/usr/local/Cellar/go/1.16.5/libexec/src/testing/testing.go:1193 +0xef fp=0xc000d67fd0 sp=0xc000d67f80 pc=0x412b30f
runtime.goexit()
/usr/local/Cellar/go/1.16.5/libexec/src/runtime/asm_amd64.s:1371 +0x1 fp=0xc000d67fd8 sp=0xc000d67fd0 pc=0x4074321
created by testing.(*T).Run
/usr/local/Cellar/go/1.16.5/libexec/src/testing/testing.go:1238 +0x2b3

goroutine 1 [chan receive]:
testing.(*T).Run(0xc000602a80, 0x5677d94, 0xf, 0x578fd78, 0x4098c01)
/usr/local/Cellar/go/1.16.5/libexec/src/testing/testing.go:1239 +0x2da
testing.runTests.func1(0xc000713080)
/usr/local/Cellar/go/1.16.5/libexec/src/testing/testing.go:1511 +0x78
testing.tRunner(0xc000713080, 0xc000c3fde0)
/usr/local/Cellar/go/1.16.5/libexec/src/testing/testing.go:1193 +0xef
testing.runTests(0xc0005490b0, 0x656a900, 0x7, 0x7, 0xc03fd7772430e868, 0x8bb4356763, 0x6633760, 0x5675a16)
/usr/local/Cellar/go/1.16.5/libexec/src/testing/testing.go:1509 +0x2fe
testing.(*M).Run(0xc000054d00, 0x0)
/usr/local/Cellar/go/1.16.5/libexec/src/testing/testing.go:1417 +0x1eb
main.main()
_testmain.go:57 +0x138

goroutine 19 [chan receive]:
k8s.io/klog/v2.(*loggingT).flushDaemon(0x6634d80)
/Users/sam/go/pkg/mod/k8s.io/klog/v2@v2.9.0/klog.go:1169 +0x8b
created by k8s.io/klog/v2.init.0
/Users/sam/go/pkg/mod/k8s.io/klog/v2@v2.9.0/klog.go:420 +0xdf

goroutine 21 [chan receive]:
k8s.io/klog.(*loggingT).flushDaemon(0x6634ca0)
/Users/sam/go/pkg/mod/k8s.io/klog@v1.0.0/klog.go:1012 +0x8b
created by k8s.io/klog.init.0
/Users/sam/go/pkg/mod/k8s.io/klog@v1.0.0/klog.go:412 +0xdc

goroutine 10 [select]:
go.opencensus.io/stats/view.(*worker).start(0xc000054880)
/Users/sam/go/pkg/mod/go.opencensus.io@v0.23.0/stats/view/worker.go:276 +0xcd
created by go.opencensus.io/stats/view.init.0
/Users/sam/go/pkg/mod/go.opencensus.io@v0.23.0/stats/view/worker.go:34 +0x68

goroutine 52 [chan receive]:
testing.tRunner.func1(0xc000602a80)
/usr/local/Cellar/go/1.16.5/libexec/src/testing/testing.go:1159 +0x2bc
testing.tRunner(0xc000602a80, 0x578fd78)
/usr/local/Cellar/go/1.16.5/libexec/src/testing/testing.go:1197 +0x125
created by testing.(*T).Run
/usr/local/Cellar/go/1.16.5/libexec/src/testing/testing.go:1238 +0x2b3

goroutine 53 [chan receive]:
testing.tRunner.func1(0xc000602c00)
/usr/local/Cellar/go/1.16.5/libexec/src/testing/testing.go:1159 +0x2bc
testing.tRunner(0xc000602c00, 0xc0016cff00)
/usr/local/Cellar/go/1.16.5/libexec/src/testing/testing.go:1197 +0x125
created by testing.(*T).Run
/usr/local/Cellar/go/1.16.5/libexec/src/testing/testing.go:1238 +0x2b3

goroutine 59 [runnable]:
gopkg.in/yaml%2ev2.(*parser).scalar(0xc000f03200, 0xc000f03206)
/Users/sam/go/pkg/mod/gopkg.in/yaml.v2@v2.4.0/decode.go:194 +0xd4
gopkg.in/yaml%2ev2.(*parser).parse(0xc000f03200, 0x6)
/Users/sam/go/pkg/mod/gopkg.in/yaml.v2@v2.4.0/decode.go:145 +0x111
gopkg.in/yaml%2ev2.(*parser).mapping(0xc000f03200, 0xc00169c509)
/Users/sam/go/pkg/mod/gopkg.in/yaml.v2@v2.4.0/decode.go:217 +0xac
gopkg.in/yaml%2ev2.(*parser).parse(0xc000f03200, 0xc00169c540)
/Users/sam/go/pkg/mod/gopkg.in/yaml.v2@v2.4.0/decode.go:149 +0xcc
gopkg.in/yaml%2ev2.(*parser).mapping(0xc000f03200, 0x9)
/Users/sam/go/pkg/mod/gopkg.in/yaml.v2@v2.4.0/decode.go:217 +0xc5
gopkg.in/yaml%2ev2.(*parser).parse(0xc000f03200, 0xc001697e03)
/Users/sam/go/pkg/mod/gopkg.in/yaml.v2@v2.4.0/decode.go:149 +0xcc
gopkg.in/yaml%2ev2.(*parser).document(0xc000f03200, 0xc000556d03)
/Users/sam/go/pkg/mod/gopkg.in/yaml.v2@v2.4.0/decode.go:175 +0xa7
gopkg.in/yaml%2ev2.(*parser).parse(0xc000f03200, 0x0)
/Users/sam/go/pkg/mod/gopkg.in/yaml.v2@v2.4.0/decode.go:153 +0x87
gopkg.in/yaml%2ev2.unmarshal(0xc00000bb00, 0x100, 0x100, 0x53999a0, 0xc001691ac0, 0x5432400, 0x0, 0x0)
/Users/sam/go/pkg/mod/gopkg.in/yaml.v2@v2.4.0/yaml.go:142 +0x24a
gopkg.in/yaml%2ev2.Unmarshal(0xc00000bb00, 0x100, 0x100, 0x53999a0, 0xc001691ac0, 0xc000232fd0, 0xc000c5b3f0)
/Users/sam/go/pkg/mod/gopkg.in/yaml.v2@v2.4.0/yaml.go:81 +0x58
sigs.k8s.io/yaml.yamlToJSON(0xc00000bb00, 0x100, 0x100, 0xc000c5b408, 0x578f128, 0xb05ad38, 0x8, 0x8, 0x6f785b8, 0x8)
/Users/sam/go/pkg/mod/sigs.k8s.io/yaml@v1.2.0/yaml.go:124 +0x73
sigs.k8s.io/yaml.yamlUnmarshal(0xc00000bb00, 0x100, 0x100, 0x53a6fa0, 0xc0046b0410, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
/Users/sam/go/pkg/mod/sigs.k8s.io/yaml@v1.2.0/yaml.go:53 +0x12a
sigs.k8s.io/yaml.Unmarshal(...)
/Users/sam/go/pkg/mod/sigs.k8s.io/yaml@v1.2.0/yaml.go:36
github.com/instrumenta/kubeval/kubeval.validateResource(0xc00000bb00, 0x100, 0x100, 0xc000cd9410, 0xc000e8e210, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
/Users/sam/go/pkg/mod/github.com/instrumenta/kubeval@v0.16.1/kubeval/kubeval.go:118 +0x109
github.com/instrumenta/kubeval/kubeval.ValidateWithCache(0xc00000bb00, 0x100, 0x100, 0xc000cd9410, 0xc000c5be78, 0x1, 0x1, 0x0, 0x0, 0x0, ...)
/Users/sam/go/pkg/mod/github.com/instrumenta/kubeval@v0.16.1/kubeval/kubeval.go:331 +0x99a
myproject.com/cmd/parallel_test.TestValidSchema.func1(0xc000603500, 0xc0006660d8)
/Users/sam/myproject/cmd/another_test.go:176 +0x269
myproject.com/pkg/testing.runTestCase.func1(0xc000603500)
/Users/sam/myproject/pkg/testing/testcase.go:125 +0x42
testing.tRunner(0xc000603500, 0xc000d9c040)
/usr/local/Cellar/go/1.16.5/libexec/src/testing/testing.go:1193 +0xef
created by testing.(*T).Run
/usr/local/Cellar/go/1.16.5/libexec/src/testing/testing.go:1238 +0x2b3

goroutine 58 [sync.Cond.Wait]:
sync.runtime_notifyListWait(0xc000fa41a0, 0x0)
/usr/local/Cellar/go/1.16.5/libexec/src/runtime/sema.go:513 +0xf8
sync.(*Cond).Wait(0xc000fa4190)
/usr/local/Cellar/go/1.16.5/libexec/src/sync/cond.go:56 +0x99
net/http.(*http2pipe).Read(0xc000fa4188, 0xc001015000, 0x1000, 0x1000, 0x0, 0x0, 0x0)
/usr/local/Cellar/go/1.16.5/libexec/src/net/http/h2_bundle.go:3515 +0x97
net/http.http2transportResponseBody.Read(0xc000fa4160, 0xc001015000, 0x1000, 0x1000, 0x0, 0x0, 0x0)
/usr/local/Cellar/go/1.16.5/libexec/src/net/http/h2_bundle.go:8611 +0xaf
bufio.(*Reader).Read(0xc001620c00, 0xc001394080, 0xa, 0x200, 0xc00138c880, 0x400d505, 0x5479920)
/usr/local/Cellar/go/1.16.5/libexec/src/bufio/bufio.go:227 +0x222
io.ReadAtLeast(0x58f3420, 0xc001620c00, 0xc001394080, 0xa, 0x200, 0xa, 0xa9d7188, 0x0, 0x1000)
/usr/local/Cellar/go/1.16.5/libexec/src/io/io.go:328 +0x87
io.ReadFull(...)
/usr/local/Cellar/go/1.16.5/libexec/src/io/io.go:347
compress/gzip.(*Reader).readHeader(0xc001394000, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
/usr/local/Cellar/go/1.16.5/libexec/src/compress/gzip/gunzip.go:175 +0xd6
compress/gzip.(*Reader).Reset(0xc001394000, 0xc0884e8, 0xc000fa4160, 0x54dd6a0, 0x200)
/usr/local/Cellar/go/1.16.5/libexec/src/compress/gzip/gunzip.go:113 +0x166
compress/gzip.NewReader(0xc0884e8, 0xc000fa4160, 0xc000fa4160, 0xc0884e8, 0xc000fa4160)
/usr/local/Cellar/go/1.16.5/libexec/src/compress/gzip/gunzip.go:94 +0x58
net/http.(*http2gzipReader).Read(0xc002e19d10, 0xc001378600, 0x200, 0x200, 0xc003424000, 0xc000b14000, 0x0)
/usr/local/Cellar/go/1.16.5/libexec/src/net/http/h2_bundle.go:9074 +0xca
io.ReadAll(0xc0884c8, 0xc002e19d10, 0xc002e19d10, 0xc0884c8, 0xc002e19d10, 0x0, 0xc0011c4410)
/usr/local/Cellar/go/1.16.5/libexec/src/io/io.go:633 +0xdf
io/ioutil.ReadAll(...)
/usr/local/Cellar/go/1.16.5/libexec/src/io/ioutil/ioutil.go:27
github.com/xeipuuv/gojsonschema.(*jsonReferenceLoader).loadFromHTTP(0xc0009dc3c0, 0xc0011c4410, 0x49, 0x0, 0x0, 0x20, 0x6f90548)
/Users/sam/go/pkg/mod/github.com/xeipuuv/gojsonschema@v0.0.0-20180816142147-da425ebb7609/jsonLoader.go:179 +0x235
github.com/xeipuuv/gojsonschema.(*jsonReferenceLoader).LoadJSON(0xc0009dc3c0, 0xc0011c42d0, 0x49, 0x592d2a8, 0xc0009dc3c0)
/Users/sam/go/pkg/mod/github.com/xeipuuv/gojsonschema@v0.0.0-20180816142147-da425ebb7609/jsonLoader.go:156 +0x247
github.com/xeipuuv/gojsonschema.(*schemaPool).GetDocument(0xc0004ca180, 0xc0003c7b00, 0x0, 0x0, 0x0, 0x100000001, 0x0, 0x0, 0xc00138d218)
/Users/sam/go/pkg/mod/github.com/xeipuuv/gojsonschema@v0.0.0-20180816142147-da425ebb7609/schemaPool.go:184 +0x1eb
github.com/xeipuuv/gojsonschema.(*SchemaLoader).Compile(0xc00138d290, 0x592d2a8, 0xc0009dc2e0, 0x4010018, 0x20, 0x54c8100)
/Users/sam/go/pkg/mod/github.com/xeipuuv/gojsonschema@v0.0.0-20180816142147-da425ebb7609/schemaLoader.go:77 +0x237
github.com/xeipuuv/gojsonschema.NewSchema(0x592d2a8, 0xc0009dc2e0, 0xc001ac6580, 0x7, 0xc001ac6568)
/Users/sam/go/pkg/mod/github.com/xeipuuv/gojsonschema@v0.0.0-20180816142147-da425ebb7609/schema.go:49 +0x85
github.com/instrumenta/kubeval/kubeval.downloadSchema(0xc00138d508, 0xc000cd9410, 0xc000e8e210, 0x5671330, 0xa, 0xc000de5288)
/Users/sam/go/pkg/mod/github.com/instrumenta/kubeval@v0.16.1/kubeval/kubeval.go:217 +0x32a
github.com/instrumenta/kubeval/kubeval.validateAgainstSchema(0x5456040, 0xc002fc74a0, 0xc00138d508, 0xc000cd9410, 0xc000e8e210, 0x0, 0x0, 0x0, 0x0, 0x0)
/Users/sam/go/pkg/mod/github.com/instrumenta/kubeval@v0.16.1/kubeval/kubeval.go:169 +0x46
github.com/instrumenta/kubeval/kubeval.validateResource(0xc001932e00, 0xe8, 0x100, 0xc000cd9410, 0xc000e8e210, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
/Users/sam/go/pkg/mod/github.com/instrumenta/kubeval@v0.16.1/kubeval/kubeval.go:159 +0x2f7
github.com/instrumenta/kubeval/kubeval.ValidateWithCache(0xc001932e00, 0xe8, 0x100, 0xc000cd9410, 0xc00138de78, 0x1, 0x1, 0x0, 0x0, 0x0, ...)
/Users/sam/go/pkg/mod/github.com/instrumenta/kubeval@v0.16.1/kubeval/kubeval.go:331 +0x99a
myproject.com/cmd/parallel_test.TestValidSchema.func1(0xc000603380, 0xc0006660c0)
/Users/sam/myproject/cmd/another_test.go:176 +0x269
myproject.com/pkg/testing.runTestCase.func1(0xc000603380)
/Users/sam/myproject/pkg/testing/testcase.go:125 +0x42
testing.tRunner(0xc000603380, 0xc000d9c020)
/usr/local/Cellar/go/1.16.5/libexec/src/testing/testing.go:1193 +0xef
created by testing.(*T).Run
/usr/local/Cellar/go/1.16.5/libexec/src/testing/testing.go:1238 +0x2b3
```