Add a function to automatically bind casual commands in mode-specific maps

Casual is a great addition but it is tedious to have to add a binding for it to every mode that it supports. IMO, it would be more convenient if there was a function like this that could be used in a user's Emacs configuration.

  ;; Binds M-C to the appropriate casual command in the map for each mode supported by casual.
  (casual-bind-key "M-C")