add custom var which-key-format-desc-first

[zhaojiangbin]

An option to format the key bindings as

find-file -> C-x C-f

This fits well with which-key-description-order.

Pull requests are welcome as long as the following apply.

1. The issue you are fixing or feature you are adding is clearly described and/or referenced in the pull request or github issue.
2. Since which-key is on GNU ELPA, any legally significant changes must have their copyright assigned to the FSF (more info). If you have not done so and would like to assign copyright, please see the request form. This process is easy, but can be slow.

[justbur]

Have you filed papers with the FSF?

[zhaojiangbin]

Have you filed papers with the FSF?

I just sent FSF the email to request the assignment form.

[zhaojiangbin]

@justbur I have got the paper work done. FSF sent me the fully signed PDF. What's next?

[justbur]

@zhaojiangbin ok, good.

I think the best way to add this option is to modify which-key--format-and-replace