Allow a sexp as head's CMD paramater

* hydra.el (hydra--make-callable): New function.
(defhydra): Use `hydra--make-callable'. Now, head's CMD is either:
command name, nil, a sexp. In case of a sexp, it will be wrapped
unevaluated in an interactive lambda. You can use a `progn' to have many
statements in the sexp.

Fixes #25.

Example:

    (defhydra hydra-launcher (:color blue)
       "Launch"
       ("h" man "man")
       ("r" (browse-url "http://www.reddit.com/r/emacs/") "reddit")
       ("w" (browse-url "http://www.emacswiki.org/") "emacswiki")
       ("s" shell "shell")
       ("q" nil "cancel"))
    (global-set-key (kbd "C-c r") 'hydra-launcher/body)

Author: abo-abo

hydra.el

@@ -5,7 +5,7 @@
 ;; Author: Oleh Krehel <ohwoeowho@gmail.com>
 ;; Maintainer: Oleh Krehel <ohwoeowho@gmail.com>
 ;; URL: https://github.com/abo-abo/hydra
-;; Version: 0.8.1
+;; Version: 0.9.0
 ;; Keywords: bindings
 ;; Package-Requires: ((cl-lib "0.5"))
 
@@ -177,6 +177,16 @@ It's possible to set this to nil.")
       (and (consp x)
            (memq (car x) '(function quote)))))
 
+(defun hydra--make-callable (x)
+  "Generate a callable symbol from X.
+If X is a function symbol or a lambda, return it.  Otherwise, it
+should be a single statement. Wrap it in an interactive lambda."
+  (if (or (symbolp x) (functionp x))
+      x
+    `(lambda ()
+       (interactive)
+       ,x)))
+
 (defun hydra--head-property (h prop &optional default)
   "Return for Hydra head H the value of property PROP.
 Return DEFAULT if PROP is not in H."
@@ -412,7 +422,7 @@ except a blue head can stop the Hydra state.
        ,@(cl-mapcar
           (lambda (head name)
             (hydra--make-defun
-             name (cadr head) (hydra--color head body-color)
+             name (hydra--make-callable (cadr head)) (hydra--color head body-color)
              (format "%s\n\nCall the head: `%S'." doc (cadr head))
              hint keymap
              body-color body-pre body-post))