`magit-key-mode-add-option': rename to `magit-key-mode-toggle-option'

Because it *toggles* instead of *adding* OPTION-NAME.

Signed-off-by: Pieter Praet <pieter@praet.org>
Merge pull request #929 from praet/key-mode-toggle

`magit-key-mode-add-option': rename to `magit-key-mode-toggle-option'