New `path` parameter in `generate`.

I added the posibility to give a specific path to `generate` so it works as 

    git diff [options] [<commit>] [--] [<path>...]

So the new command definition would be:

    diffscuss generate [<options>] <git_revision_range> [--] [<path>...]

This pull requests includes tests.
Awesome work guys! both in diffscuss and impermagit =)