Add a link to the TODO specified by pygemony

[tjdevries]

This would be classified as an enhancement (I believe).

The idea is that when you have a "TODO" that is sent to GitHub by pygemony, there would be a link in the TODO message that could send you directly to the line that the message is about. I know it says the file and the location, but it might be nice to link to the specific file (perhaps even optionally with the commit that the TODO originally came from).

[Ianleeclark]

I like it: some potential short-falls that I can think of are branch detection and pygemony doesn't really have anything which detects branches yet, but branch detection is as easy as getting the ref from .git/HEAD in the current directory.