Add a hook so packages can support more version control systems

The project API currently only supports Git. This diff makes it possible for 3rd-party packages to add support for other version control systems, addressing issue https://github.com/atom/atom/issues/4138.

Here's an example of a preliminary package that adds support for Mercurial: https://github.com/oclbdk/vcs-hg

Here's a demo of it in action:

![vcs-hg demo](http://i.imgur.com/3vOhrml.gif)