Support for outputting a file per kind

[akselleirv]

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
├── <group one>/v1alpha1/MyResource.md
├── <group one>/v1beta1/MyResource.md
└── <group two>/v1alpha1/MyOtherResource.md