Add a new `overlay` command to be able to apply OpenAPI overlays on given API document

[paulRbr]

Together with the guide written by Phil on OpenAPI overlays, we thought it would be a good idea to release a new bump overlay command in the CLI in order to test the new OpenAPI Overlay specification.

This WIP change is based on @lornajane's work on overlays with a few changes here and there.

I've published the resulting built package from this branch as a beta version on npmjs registry so you can test a beta version of Bump CLI installable with:

npm install bump-cli@beta

---

Example usage

• Wrong overlay file

  > bump overlay ../bump/doc/api/v1/openapi.v3.yml ../bump/doc/api/v1/openapi.v3.yml
  ›   Error: ../bump/doc/api/v1/openapi.v3.yml does not look like an OpenAPI overlay

• Correct overlay file

  bump overlay ../bump/doc/api/v1/openapi.v3.yml ../bump/doc/api/v1/overlay.yaml > ../bump/doc/api/v1/openapi.v3.overlayed.yml
  bump deploy  ../bump/doc/api/v1/openapi.v3.overlayed.yml

Note: the input API document file name is used to detect the target output overlayed document (Json or Yaml)

• With the explicit --out argument

bump overlay ../bump/doc/api/v1/openapi.v3.yml ../bump/doc/api/v1/overlay.yaml --out ../bump/doc/api/v1/openapi.v3.overlayed.yml

• Applying an overlay right before a deployment

bump deploy ../bump/doc/api/v1/openapi.v3.yml --overlay ../bump/doc/api/v1/overlay.yaml --doc my-doc --token --my-token

[philsturgeon]

Can we pop in a Thanks section on the README and put Lorna Mitchel on there?

We've done this for Spectral, Prism, and I recommend it for any tool which has substantial contribution from a single contributor, like this.

https://github.com/stoplightio/spectral#-thanks

[paulRbr]

@scharrier following your review, I've moved things around to make it clearer (I think). Basically all the “command” related feature is in the new commands/overlay.ts file while all the things around the definition (format, applying overlay...) is in the APIdefinition model.

I think it makes more sense like this. Let me know if it suits you!