Allow cloning from GitHub without having to prefix url

[hwittenborn]

Summary

The GitHub integration in Atom currently supports cloning from anywhere, but you have to add the full URL.

I'd like to just have to type my username and repository name(i.e. atom/github) and have it pull said repository from GitHub.

An alternative could be listing the repositories for an account(if you're signed into GitHub) and then picking a repository from that.

Motivation

I just don't like having to get the full URL from GitHub. Just having to type the user/org and the repo would help make things quicker when setting up new repos as well.