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?

Unrelated but lines 34/35 need to be removed any, as I do that filtering (again) in `reduceNamespaces`, which I only added last night. That doesn't change your original point though.