Added some kind of warning when a folder is not a git repository

Hi,
found your tool today. It's nice! Thanks.

When I have a folder with repositories in it, I only have repositories in it. But I sometimes forget to git init ;-) Therefore I changed clustergit to print a warning when a folder is not under git version control. If you like, I can add an option to make this feature optional. What do you think?
Cheers,
Ben