Add a kill buffer and implement yanking

Ok, one last patch to the REPL to implement yanking.  Binds control-y to yank. The kill buffer (not a real kill-ring) is stored in MIState.

I think the new REPL is now at a feature parity with my most-used readline keybindings and behaviors.  It's been wonderful to work with… and so simple to implement these new features!

cc: @nolta, @loladiro