Added some kind of warning when a folder is not a git repository
Improvements after Micheals response

* Add option --warn-unversioned, disabled by default
* Changed color of unversioned directories to WARNING
* Only warn if it is a directory
Showing the branch a repository is currently on
warn_unversioned: default to False instead of []
document --warn-unversioned
Merge branch 'benediktarnold-no_repository'
Merge branch 'show_branch' of github.com:benediktarnold/clustergit into benediktarnold-show_branch
Merge branch 'benediktarnold-show_branch'