Two New Features

Hey there. Here are two features I've wanted for a little while (sorry, probably should have put them in separate branches but they're small and you should be able to cherry pick if you don't want both).

One of them adds support for bpython-urwid (I think maybe I'll experiment later with getting this to run inside pudb's mainloop, since that can open up some possibilities).

The other adds support for specifying an arbitrary expression to use as the stringifier for each variable.