Add repo shortcuts (github, bitbucket, gitlab)

Adds a github prefixed shortcut and matching bitbucket and gitlab shortcuts. Adds detection and folding of github/bitbucket/gitlab git urls into shortcuts to allow consumers to do special handling without having to special case.
A habit I picked up from... *googles* ah yes, Perl Best Practices, [the original argument is here](http://books.google.com/books?id=gJf9tI2mytIC&pg=PT269&dq=%22Perl+Best+Practices%22+Metacharacters&hl=en&sa=X&ei=ip9JVNCYJK7IsASGq4D4DA&ved=0CB8Q6AEwAA#v=onepage&q=%22Perl%20Best%20Practices%22%20Metacharacters&f=false).