Limit hotkey to be platform-specific
Limit hotkey to be platform-specific

Previously both ctrl+k and cmd+k were useable on Mac.  On
Windows, meta(windows?)+k and ctrl+k were theoretically useable.
Now only ctrl+k works on Windows and only cmd+k works on Mac.

Also, I threw in a new feature: the hotkey letter can now be changed
when initializing the quick switcher.

lightster/quick-switcher,lightster/quick-switcher,lightster/quick-switcher