Support reprogrammable keys

Created by: Lekensteyn

As requested in http://superuser.com/q/466253/47108.

The keyboards seem to allow changing the functionality of Fn + Fx keys, instead of "Email", another event is generated (I haven't checked whether this is a regular keymapping that is sent to the input layer or only a HID++ notification).

I will probably start working on this next week.