"Improve this on github" link

[drbrain]

Allow rdoc to know the github repository and provide a link to the file and line where the method starts

Rdoc already has this information so it should not be hard to provide just a link.

[zzak]

sdoc currently does this
see https://github.com/voloko/sdoc/blob/master/lib/sdoc/github.rb