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 
Will probably need batch, simply because we will run out of command-line buffer.  In terms of format we will need a source and a destination namespace attribute. Maybe opt[(String, String)], via "src-namespace:attribute" "trg-namespace:attribute".