VCS hosting abbreviations

This lets you use abbreviated URLs for templates on common hosting sites, like `gh:audreyr/cookiecutter-pypackage`, as discussed on issue #246. gh expands to Github, and bb to Bitbucket.

I haven't tried to make this configurable by the user at present - that seems rather less useful to me. However, technically it would be fairly straightforward to add those if there's demand.
No special reason - this seemed like the most obvious way to write it, but I'm happy to do it that way if people prefer.