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 
> Is this possible? It seems like it should be an error condition.

I tried something to improve the state and `PriorityTagDeserializer` on the other PR (but as a separate commit if it wasn't what you had in mind):

https://github.com/ambiata/ivory/commit/b18f74adde7aa540f24dc1c99450e1877d516545