Feature: support checking only the "files changed" or "lines changed"

[Raynos]

We should add a mode where we can pass a series of files to check.

This would allow us to check only files changed in git. This can be done with jsig $(git ...). But less just add a jsig git-files HEAD..master sub command.

Would also be nice to have a secondary mode that checks only lines changed. This is best done by running the checker on the entire file but filtering the errors by the line numbers based on changed files. This can only be done by a sub command like jsig git-lines HEAD..master