Feature: Add targeting for groups in regular commands

[LordMike]

So f.ex. today, we can do: mu st repo:repo1,repo2.. But we cannot add repo1, repo2 to a group x, and then do mu st repo:x.

I suggest that the repo: filter should first check for repositories by name, and if they don't exist, check for groups of the same name. If not already the case, groups should not be allowed to be named the same as repositories.

This will also allow me to do: mu st repo:x,other-repo to target repo1, repo2 and other-repo.

Note: Github is having issues today, so I may have double-posted this as #44. If that's the case, close this one.

[fabioz]

Yeap, GitHub seems to be having issues today... closing this one.