Allow heads to conditionally exit

Allow heads to conditionally exit

* hydra.el (hydra-deactivate): New defvar.
(hydra-set-transient-map): When `hydra-deactivate' is set, quit.
(hydra-disable): Make sure that `hydra-deactivate' is reset back to nil.

Fixes #115

Example: zoom in at most 5 times, then quit.

(defvar hydra-zoom-amount 1)

(defhydra hydra-zoom (global-map "<f2>")
  "zoom"
  ("g"
   (if (>= hydra-zoom-amount 5)
       (progn
         (setq hydra-zoom-amount 1)
         (setq hydra-deactivate t))
     (cl-incf hydra-zoom-amount)
     (call-interactively 'text-scale-increase))
   "in")
  ("l" text-scale-decrease "out"))