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 it possible to only get the sizes of namespaces in the mapping instead of reading all and filtering?

Almost certainly. I wasn't sure whether the general approach/idea of `allNamespaceSizes` is even correct, but if it is I'm happy to make it more efficient.