Allow customizing hooks defined via define-minor-mode

* lisp/emacs-lisp/easy-mmode.el (define-minor-mode): Allow using
Customize on the hooks (bug#10773).

lisp/emacs-lisp/easy-mmode.el

@@ -335,6 +335,9 @@ or call the function `%s'."))))
 No problems result if this variable is not bound.
 `add-hook' automatically binds it.  (This is true for all hook variables.)"
                        modefun)))
+       ;; Allow using using `M-x customize-variable' on the hook.
+       (put ',hook 'custom-type 'hook)
+       (put ',hook 'standard-value (list nil))
 
        ;; Define the minor-mode keymap.
        ,(unless (symbolp keymap)	;nil is also a symbol.