Allow Mac Command key in addition to Ctrl for keyboard shortcuts

[jaredkrinke]

• [x] Test hitting enter on button
• [x] Ctrl+K Ctrl+U/C
• [x] Ctrl+M for tab insert mode
• [x] Enter on breakpoint
• [x] Ctrl+Enter/Shift+Enter
• [x] Ctrl+.
• [x] Ctrl+Z/Y while running

Some Mac keyboards don't have a Ctrl key, apparently. Fortunately, Command appears as metaKey, so it's easy to just add it as an option.