magit-key-mode.el: make faces customizable

As requested [1] by  Phil Jackson <phil@shellarchive.co.uk>.

[1] mid:"CAFf4W1ZXpwRtPLCMULSm8h50fJHUvGzUtOw32xrzhMVgG870Zg@mail.gmail.com"

Signed-off-by: Pieter Praet <pieter@praet.org>