More ways to filter repositories

[luolong]

I think it is great that I can filter my repositories by tags, but quite often, when working with multiple repositories, I need to run my git command on a subset of the tagged repositories.

Couple of specific points of interest that might be good candidates for filtering:

1. status of the repo: modified/unmodified, ahead or behind
2. name of the active branch

First one could be triggered with -s <modified | clean | ahead | behind>.

Second could be triggered by -b <branch_name>

[rocketraman]

@luolong The workaround here may help you: https://github.com/mixu/gr/issues/93