Add support for Fossil

[MistressRemilia]

I'd like to see support for Fossil added to Shards. The last I checked, Go's go get and friends support Fossil repositories, so seeing the same sort of feature in Shards would be nice. I'm willing to do a PR for this if that's OK.

I also almost never work directly with Git or Mercurial in my day-to-day work, and so I normally resort to modifying shards.yml to point to local directories. Not having to do this would be nice as well :-P

[straight-shoota]

Sure. I don't think there's any reason not to add fossil support. It's not on our priority list, but contributions are welcome.

Make sure to look at #458 for inspiration on how to add a new resolver.