Add :pre and :post clauses to Hydra body

* hydra.el (defhydra): the PLIST part of BODY argument now recognizes
  :pre and :post keys. These should be single Elisp statements,
  wrappable in a lambda. When you need more than one statement, use a
  `progn'.

:pre will be called by `hydra-foo/body', as well as by all heads.
:post will be called by the blue heads, as well as on Hydra termination
by a command that isn't a head.

Fixes #16.

An Example:

    (global-set-key
     (kbd "C-z")
     (defhydra hydra-vi
         (:pre
          (set-cursor-color "#40e0d0")
          :post
          (set-cursor-color "#ffffff"))
       "vi"
       ("l" forward-char)
       ("h" backward-char)
       ("j" next-line)
       ("k" previous-line)
       ("q" nil "quit")))

hydra.el

@@ -5,7 +5,7 @@
 ;; Author: Oleh Krehel <[email protected]>
 ;; Maintainer: Oleh Krehel <[email protected]>
 ;; URL: https://github.com/abo-abo/hydra
-;; Version: 0.6.1
+;; Version: 0.7.0
 ;; Keywords: bindings
 ;; Package-Requires: ((cl-lib "0.5"))
 
@@ -116,7 +116,7 @@
     (define-key map [kp-9] 'hydra--digit-argument)
     (define-key map [kp-subtract] 'hydra--negative-argument)
     map)
-  "Keymap that all Hydras inherit. See `universal-argument-map'.")
+  "Keymap that all Hydras inherit.  See `universal-argument-map'.")
 
 (defvar hydra-curr-map
   (make-sparse-keymap)
@@ -130,7 +130,7 @@
                      (if (eq arg '-)
                          (list -4)
                        '(4))))
-  (hydra-set-transient-map hydra-curr-map))
+  (hydra-set-transient-map hydra-curr-map t))
 
 (defun hydra--digit-argument (arg)
   "Forward to (`digit-argument' ARG)."
@@ -298,21 +298,28 @@ in turn can be either red or blue."
                          'red
                        (or (plist-get (cddr body) :color)
                            'red)))
+         (body-pre (plist-get (cddr body) :pre))
+         (body-post (plist-get (cddr body) :post))
          (method (if (hydra--callablep body)
                      body
                    (car body)))
          (hint (hydra--hint docstring heads body-color))
          (doc (hydra--doc body-key body-name heads)))
+    (when (and (or body-pre body-post)
+               (version< emacs-version "24.4"))
+      (error "At least Emacs 24.4 is needed for :pre and :post"))
     `(progn
        ,@(cl-mapcar
           (lambda (head name)
             `(defun ,name ()
                ,(format "%s\n\nCall the head: `%S'." doc (cadr head))
                (interactive)
+               ,@(if body-pre (list body-pre))
                ,@(if (eq (hydra--color head body-color) 'blue)
                      `((hydra-disable)
                        ,@(unless (null (cadr head))
-                                 `((call-interactively #',(cadr head)))))
+                                 `((call-interactively #',(cadr head))))
+                       ,@(if body-post (list body-post)))
                      `((catch 'hydra-disable
                          (hydra-disable)
                          (condition-case err
@@ -325,7 +332,10 @@ in turn can be either red or blue."
                          (when hydra-is-helpful
                            (message ,hint))
                          (setq hydra-last
-                               (hydra-set-transient-map (setq hydra-curr-map ',keymap) t)))))))
+                               (hydra-set-transient-map
+                                (setq hydra-curr-map ',keymap)
+                                t
+                                ,@(if body-post `((lambda () ,body-post))))))))))
           heads names)
        ,@(unless (or (null body-key)
                      (null method)
@@ -347,10 +357,14 @@ in turn can be either red or blue."
        (defun ,body-name ()
          ,doc
          (interactive)
+         ,@(if body-pre (list body-pre))
          (when hydra-is-helpful
            (message ,hint))
          (setq hydra-last
-               (hydra-set-transient-map ',keymap t))))))
+               (hydra-set-transient-map
+                ',keymap
+                t
+                ,@(if body-post `((lambda () ,body-post)))))))))
 
 (provide 'hydra)