Handle all extended keymaps

This change lets god-mode handle all extended keymaps, instead of just those explicitly defined in `god-mode-interpret-key`. To do so, it sacrifices two things:

 - no differentiation in error messages between unknown key binding and extended key binding.
 - Prompt is "C-x" and "M-g", not "x" and "g"

Hopefully those are worthwhile trade-offs. Thanks for taking a look!