Add 'list' command to list all files owned for a given owner

[rstawarz]

Adds a simple list command to verify ownership of files for a given owner. I found this useful when making changes to CODEOWNERS in large code bases with complex rules.

example usage: codeowners-checker list @owner1

[rstawarz]

@jonatas - Thanks! I wasn't sure how this fit into the tool, happy to adjust if you have a better vision of how this should work, just let me know

[jonatas]

@rstawarz please add some high level documentation on the README too.

It looks a bit weird that we have list now and filter command looks very similar. I was looking for a way to unify them like:

code-owners-checker list --only-changes
code-owners-checker list --owner=some-owner

It seems like list and filter are almost interchangeable and we can even combine them.

code-owners-checker list --only-changes --owner=some-owner

I'm just thinking aloud here. @ekadlecova, @dennissivia, @bamorim any thoughts about it?