Add settings for GitClean alias

[steverichey]

I would like to use the GitClean alias to reset most of a repo, with the exception of a build folder. Currently, I don't think Cake.Git supports this. I would recommend a GitCleanSettings object that can be passed to GitClean with properties for the git clean flags.

[nils-a]

Cake.Git is based on LibGit2Sharp, so we could only provide what LibGit2Sharp enables us to do.
Currently there is no clean in LibGit2Sharp and GitClean uses IRepository.RemoveUntrackedFiles which has no options available.