[WIP][RFC] Clump.source zip

This branch contains some hacky/incomplete stuff but I just wanted to push it for comment.

@fwbrasil I think it's nice to have this convenience version of ClumpSource for repositories that have that can fulfill the zip contract.

However, I had to make a bit of a mess to get everything compiling and running and backwards compatible with the existing `source` methods which take `Set`s and `Iterable`s. Ultimately it would be nice to be able to pass any subtype of Iterable to these methods, but I had a go and my knowledge of scala generics isn't good enough yet. I might have a go in the next few days if I'm not too busy but it's hard to know.

Just putting this out there for comment regarding `sourceZip` and regarding a more generic implementation for the other `source` methods