Add support for git-clean

[tonybruess]

Please forgive me if I'm wrong, but I can't find support for http://www.kernel.org/pub/software/scm/git/docs/git-clean.html in rugged right now.

[gcapizzi]

Can't find it on libgit2 either (http://libgit2.github.com/libgit2), so i guess you should open an issue there first.

[clns]

See Can I do git-clean with libgit2? on SO. Based on that answer:

libgit2 doesn't implement this itself. It's a porcelain command, which deals with interaction with the user. It also deals primarily with files which are of no interest to the Git repository.

You can run status to see which files are untracked and then remove whichever ones the version of git-clean would have removed through the usual means available in your programming language.

[tonybruess]

Should this be closed then?