Merge section in docfx.json does not support more than one merge operation

**DocFX Version Used**: 2.28.2

**Steps to Reproduce**:

I am combining two C# projects into a single web site, so I use the following `docfx.json`,
```
{
"metadata": [
{
"src": [
{
"files": [
"sharpsnmplib/SharpSnmpLib/*.csproj"
],
"exclude": [
"**/bin/**",
"**/obj/**"
],
"cwd": "."
}
],
"properties": {
"TargetFramework": "net452"
},
"dest": "obj/lib/10.0.2_net452/api",
"shouldSkipMarkup": true
},
{
"src": [
{
"files": [
"sharpsnmplib/SharpSnmpLib/*.csproj"
],
"exclude": [
"**/bin/**",
"**/obj/**"
],
"cwd": "."
}
],
"properties": {
"TargetFramework": "netstandard1.3"
},
"dest": "obj/lib/10.0.2_netstandard1.3/api",
"shouldSkipMarkup": true
},
{
"src": [
{
"files": [
"sharpmibsuite/SharpSnmpPro.Mib/*.csproj"
],
"exclude": [
"**/bin/**",
"**/obj/**"
],
"cwd": "."
}
],
"properties": {
"TargetFramework": "net452"
},
"dest": "obj/pro/2.0.0_net452/api",
"shouldSkipMarkup": true,
"filter": "filterConfig.yml"
},
{
"src": [
{
"files": [
"sharpmibsuite/SharpSnmpPro.Mib/*.csproj"
],
"exclude": [
"**/bin/**",
"**/obj/**"
],
"cwd": "."
}
],
"properties": {
"TargetFramework": "netstandard2.0"
},
"dest": "obj/pro/2.0.0_netstandard2.0/api",
"shouldSkipMarkup": true,
"filter": "filterConfig.yml"
}
],
...
```
Then I can use the following to merge the documents for sharpsnmplib,
```
"merge": {
"content": [
{
"files": "*.yml",
"src": "obj/lib/10.0.2_netstandard1.3/api"
},
{
"files": "*.yml",
"src": "obj/lib/10.0.2_net452/api"
}
],
"fileMetadata": {
"platform": {
"obj/lib/10.0.2_netstandard1.3/api/*.yml": [
"netstandard1.3"
],
"obj/lib/10.0.2_net452/api/*.yml": [
"net452"
]
}
},
"dest": "obj/lib/10.0.2/api"
},
```

> I hit a null reference exception (which should have been fixed 17 days ago, but not shipped in this release).

**Expected Behavior**:
I should be able to specify another merge operation, to merge metadata for sharpmibsuite.

**Actual Behavior**:
I cannot use multiple "merge" sections, which makes docfx stop working miserably. It is surprising the docfx does not generate any error but just stops in the middle of something.

I cannot use `[]` either. The following changes in docfx.json would lead to an error,
```
"merge": [
{
"content": [
{
"files": "*.yml",
"src": "obj/lib/10.0.2_netstandard1.3/api"
},
{
"files": "*.yml",
"src": "obj/lib/10.0.2_net452/api"
}
],
"fileMetadata": {
"platform": {
"obj/lib/10.0.2_netstandard1.3/api/*.yml": [
"netstandard1.3"
],
"obj/lib/10.0.2_net452/api/*.yml": [
"net452"
]
}
},
"dest": "obj/lib/10.0.2/api"
},
{
"content": [
{
"files": "*.yml",
"src": "obj/pro/2.0.0_netstandard2.0/api"
},
{
"files": "*.yml",
"src": "obj/pro/2.0.0_net452/api"
}
],
"fileMetadata": {
"platform": {
"obj/pro/2.0.0_netstandard2.0/api/*.yml": [
"netstandard2.0"
],
"obj/pro/2.0.0_net452/api/*.yml": [
"net452"
]
}
},
"dest": "obj/pro/2.0.0/api"
}
],
```
The error message is
```
[17-12-10 11:01:08.132]Info:Config file docfx.json found, start generating metadata...
[17-12-10 11:01:08.165]Error:Cannot deserialize the current JSON array (e.g. [1,2,3]) into type 'Microsoft.DocAsCode.MergeJsonConfig' because the type requires a JSON object (e.g. {"name":"value"}) to deserialize correctly.
To fix this error either change the JSON to a JSON object (e.g. {"name":"value"}) or change the deserialized type to an array or a type that implements a collection interface (e.g. ICollection, IList) like List that can be deserialized from
a JSON array. JsonArrayAttribute can also be added to the type to force it to deserialize from a JSON array.
Path 'merge', line 82, position 12.
```
You should support multiple merge operations somehow, as you already support multiple metadata operations.