Added ability to rename features

I'm not completely satisfied with this, but wanted to get it open sooner rather than later. I'll leave a few inline comments. The branch obviously won't build until we do something with `IvoryT` (I really wish you could re-target PRs in GH)

/cc @etorreborre @markhibberd @raronson 
I think `IvoryT` is a good idea, but I suspect I might get shot down for this. I do like that the `rename` function is nice and clean. Happy to take it away though.