Add support for batch builds

I can't seem to find a way to run this against a codebuild batch job. This functionality would be great!

Example buildspec.yml

```
version: 0.2

batch:
build-graph:
- identifier: templates
buildspec: templates/buildspec.yml
```

No source file or test is named. Start by inspecting the repository's image and build configuration, then determine how the supplied buildspec.yml batch build-graph example should be supported for an AWS CodeBuild batch job; done means that example runs successfully.

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