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 
> Not 100% sure, but do you need mapping.oldFeatures here? It looks like you already remove them from the dictionary in updateDictionary

Good point. I've removed those lines anyway, which will fix that.