Adaptive prefix bindings

[drzel]

You mention in your readme that if you have C-a as your prefix then C-a a, will be last window, and if it's C-z, then C-z z, will be last window.

This doesn't appear to be working on C-s for me, but when I think about it - is this a feature you want? Prefx + s is already something else, as are many other combinations.