Optionally exclude files or directories during git clean with the git_clean_exclude config property.

This is useful for us to keep from having to download Bower libs and Node modules that are used during
our build process, since these are not installed system-wide (and instead are installed per-user).
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/).
Merge pull request #2 from Coefficient/master

Optionally exclude files or directories during git clean