Autodiscover files to validate

[anttiharju]

Hi, thanks for this awesome tool.

I've been using it in CI and locally like this:

git ls-files -z ".github/*/*.yml" "*/action.yml" | xargs -0 action-validator --verbose

This is bit of an incantation and I would love for the tool to be a bit more ergonomic. Given that the focus is on GitHub Actions, it should be safe to assume that any of the files that need validating are tracked by Git, so I think we could bake in the git ls-files -z ".github/*/*.yml" "*/action.yml", so that the tool could be used like this:

action-validator

this would be similar to how actionlint works, it can be simply used like this:

actionlint

[mpalmer]

I wouldn't say no to a PR that implemented auto-discovery, although I'm not (yet?) comfortable making auto-discovery the default -- I feel like there's potential dragons lurking behind shelling out when passed no arguments. Definitely having a -a / --auto option would be a cool addition, though.