Better which-key support

[justinbarclay]

@bbatsov, I recently started playing with which-key and noticed that some of Projectile's mappings aren't labelled.

I'm able to improve the labelling of nested maps using a which-key helper, which-key-add-keymap-based-replacements,
for example:

    (defvar projectile-other-window-map
      (let ((map (make-sparse-keymap)))
        (define-key map (kbd "a") #'projectile-find-other-file-other-window)
        (define-key map (kbd "b") #'projectile-switch-to-buffer-other-window)
        (define-key map (kbd "C-o") #'projectile-display-buffer)
        (define-key map (kbd "d") #'projectile-find-dir-other-window)
        (define-key map (kbd "D") #'projectile-dired-other-window)
        (define-key map (kbd "f") #'projectile-find-file-other-window)
        (define-key map (kbd "g") #'projectile-find-file-dwim-other-window)
        (define-key map (kbd "t") #'projectile-find-implementation-or-test-other-window)
        map))
    ...
    (which-key-add-keymap-based-replacements projectile-command-map
      "4" (cons "other-window" projectile-other-window-map)
      "5" (cons "other-frame" projectile-other-frame-map)
      "s" (cons "search" projectile-search-map))

Ending up with:

However, would you be receptive of a PR that helps improve the labelling with which-key?

P.S. Thanks for all of your work on Projectile. It's an amazing tool ❤️