Allow KeyCombos with no modifiers

[chendo]

I needed to create hot keys that had no modifiers, however the guard statement in public init?(key: Key, cocoaModifiers: NSEvent.ModifierFlags) was preventing me from doing so, and trying to work around the issue by creating my own hot keys directly with Carbon APIs caused asserts so I figured this is the better solution.

Closes #79

[gluonfield]

This should be merged.