VCS specifier

I see you have git specifiers.

It might be useful to use https://github.com/retr0h/git-url-parse to help with parsing. Despite saying it is not maintained, coala developers havent had problems getting PRs reviewed and merged, and we dont see many types of git URLs which it fails to correctly parse. (We tried most of the others we could find, and they were worse..)

But, there is also a need for hg+, svn+, etc Not as sexy as git, but still needed for compatibility with pip, and likely others.