Support for outputting a file per kind

In an internal documentation I have a file exists per resource/kind, so it would be nice to output the API references for each kind.

Example:
```
$ crd-ref-docs --source-path=./pkg/apis \
--config=config.yaml \
--renderer=markdown \
--output-path ./output \
--output-mode=kind

output
├── /v1alpha1/MyResource.md
├── /v1beta1/MyResource.md
└── /v1alpha1/MyOtherResource.md
```