add 'command' as hotkey trigger for Macs

[charliepark]

Code showing what to change to enable this here: charliepark/jquery.hotkeys@6878ec4

[charliepark]

If this is something you want to add in to the code, I'd be happy to fork/patch/submit. Just let me know.

[bencrowder]

Go for it. Let me know when it's ready and I'll merge it back in. Thanks!