Add support for bundles

[nathan-joslin]

Git bundles are an extension of pack files. Given that go-git already supports pack files via plumbing/format/packfile it would be nice to extend this functionality to support bundles.

[rikkuness]

Possible duplicate of #497 ?

[nathan-joslin]

Possible duplicate of #497 ?

IMHO, related but not duplicates. Nevertheless, I also believe the question is somewhat invalid as I do not believe git clone supports cloning from bundles or packfiles. It is to my understanding that git bundles aren't "cloned" but rather pulled into an existing repository. If one wants to re-construct a repository from a bundle you could bundle the entire repository and then do a git fetch from the bundle into an empty repo. In effect this would be a clone.

@rikkuness