Allow changes to inner keymaps to propagate correctly

This allows later calls to define-key on
`projectile-rails-mode-goto-map` and `projectile-rails-mode-run-map` to
change the keymap referenced by the inner keymap in `projectile-rails-mode-map`.
Allow modification to command keymap after prefix

Also allows alternate keybindings to reference this keymap, and to
modify it's contents.