Show missing commands for keybindings

[izuzak]

In atom/atom#9833 a user and I had problems figuring out why a keybinding didn't work. The problem was that the command which the keystroke was bound to didn't exist. Currently, the keybinding resolved doesn't visually indicate that. It would be neat if it did.

[bdowling]

This is an oldie, but I like this idea too. Easy to do if you say put a . where a : should have been in the target cmd. :)