Add support for Devil

[wesnel]

What is Devil?

Devil mode trades your comma key in exchange for a modifier-free editing experience in Emacs.

The above description comes from their README at:

https://github.com/susam/devil

Unfortunately, Devil is not currently supported by which-key. For example, when I type , x, the which-key popup for C-x does not appear. Instead, I merely see the following text in the echo area:

Devil: C-x

With this patch, I have mimicked the existing god-mode logic in which-key to intercept the current Devil prompt and spawn a which-key popup.

This feature was requested by susam/devil#3:

• https://github.com/susam/devil/issues/3