Allow turning shortcuts on and off

[guyoron]

An app may want to turn shortcuts off at particular times or on particular pages. Allow toggling the whole functionality on and off.

[jkup]

I'm interested in both this and #9. I think it'll require a significant refactor in the sense that right now this library is just an IIFE with an event listener. To be able to turn them on or off / add more, we'd need to register a namespace like jQuery / Underscore do.