Implement Stringer-interface for Key and Attribute

It would be nice if termbox understood human-readable representations of keys and colors. This would allow to display keystrokes on GUIs, load & safe hotkeys from setting files etc without digging to deeply into termbox's code. The main motivation is to prevent lestrrat/peco and all the others from re-implementing this over and over again.

I may add the other keys upon request...