Feature checkout

I've added a function that will check if a remote feature exists when you try to do a checkout. For instance, if you type "git hf feature checkout newfeature", the function "hubflow_track_repo" will execute and either return and do nothing if it exists or track the feature if it doesn't.

I added this because I could not get the command to work as described in the documentation. I've also tested this extensively together with Christopher Kvarme, whom you've had some contact with lately.