Add option to ignore non git folders

[psav]

Fixes #143

[coveralls]

Coverage increased (+0.003%) to 82.212% when pulling c75396c on psav:ignore_non_git into 0edda26 on psss:master.

[psss]

Thanks for the patch, Pete. When I was thinking about it a bit more I realized ignoring non-git directories should be the default behaviour. And unless we have a strong use case for the new option I would keep it simple. Fix pushed as aaeaf1a into devel.