Strict mode

It would be nice to have more strict mode of parsing with the option of tagging fields as required, as well as returning an error when superfluous fields are present.

Given the following struct and config `field1` is omitted and `field3` is a superfluous field.

``` go
type Section struct {
Name string `hcl:",key"`
Field1 string
Field2 string
}
```

``` hcl
section "Section1" {
field2 = "2"
field3 = "3"
}
```

The object returned is `[]main.Section{main.Section{Name:"Section1", Field1:"", Field2:"2"}}`

No files, tests, or parser entry points are named. Start by locating the Go code that decodes the shown struct and HCL configuration, then establish tests for omitted required fields and superfluous fields; done means strict parsing reports both cases while preserving the existing behavior by default.

Written by the indexing model from the issue text.