Updated with Brian's requested changes. `git_clean_exclude` is now a project option (I'm not
sure why I didn't think of this in the first place).

Note that I did not make the default `__init__()` argument `[]` as this is problematic
in Python (there are ways around this but `None` is the safest)
(see http://eli.thegreenplace.net/2009/01/16/python-insight-beware-of-mutable-default-values-for-arguments/).