Let users ignore authors

[tomice]

The git-quick-stats project allows users to ignore authors. This is useful if you have some sort of bot or automated CI pipeline take over and handle merging of commits or patches. The parent project uses a _GIT_IGNORE_AUTHORS variable to allow you to filter out authors. Example usage would be as follows:

$ export _GIT_IGNORE_AUTHORS="(author1|author2)"
$ git-quick-stats

Make sure all documentation is updated to match this feature.