Add Format → Raw command bound to ⌘K

This command handles the following four situations:

 1. No selection, caret on a word: Wraps word in back-ticks (shorthand for ⌃W + ` + “unselect”).
 2. No selection, caret not on a word: Inserts `‸` (same as pressing back-tick).
 3. Selection not wrapped in back-ticks: Wraps selection (same as pressing back-tick).
 4. Selection wrapped in back-ticks: Strips the back-ticks (same as pressing ]).