Add option to specify :hint in body

* hydra.el (hydra-plist-get-default): New defun, extended `plist-get'.
(hydra--head-property): Use `hydra-plist-get-default'.
(defhydra): The heads will inherit their hint from body :hint parameter.
They can override it, of course.

The most use you can get out of this parameter is to specify :hint nil
for hydras with a format-style docstring.

Example:

(defhydra hydra-org-template (:color blue :hint nil)
  "
_c_enter  _q_uote    _L_aTeX:
_l_atex   _e_xample  _i_ndex:
_a_scii   _v_erse    _I_NCLUDE:
_s_rc     ^ ^        _H_TML:
_h_tml    ^ ^        _A_SCII:
"
  ("s" (hot-expand "<s"))
  ("e" (hot-expand "<e"))
  ("q" (hot-expand "<q"))
  ("v" (hot-expand "<v"))
  ("c" (hot-expand "<c"))
  ("l" (hot-expand "<l"))
  ("h" (hot-expand "<h"))
  ("a" (hot-expand "<a"))
  ("L" (hot-expand "<L"))
  ("i" (hot-expand "<i"))
  ("I" (hot-expand "<I"))
  ("H" (hot-expand "<H"))
  ("A" (hot-expand "<A"))
  ("<" self-insert-command "ins")
  ("o" nil "quit"))

Some setup:

(defun hot-expand (str)
  "Expand org template."
  (insert str)
  (org-try-structure-completion))
(define-key org-mode-map "<"
  (lambda () (interactive)
     (if (looking-back "^")
         (hydra-org-template/body)
       (self-insert-command 1))))

hydra.el

@@ -252,13 +252,21 @@ should be a single statement.  Wrap it in an interactive lambda."
        (interactive)
        ,x)))
 
+(defun hydra-plist-get-default (plist prop default)
+  "Extract a value from a property list.
+PLIST is a property list, which is a list of the form
+\(PROP1 VALUE1 PROP2 VALUE2...).
+
+Return the value corresponding to PROP, or DEFAULT if PROP is not
+one of the properties on the list."
+  (if (memq prop plist)
+      (plist-get plist prop)
+    default))
+
 (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."
-  (let ((plist (cl-cdddr h)))
-    (if (memq prop h)
-        (plist-get plist prop)
-      default)))
+  (hydra-plist-get-default (cl-cdddr h) prop default))
 
 (defun hydra--aggregate-color (head-color body-color)
   "Return the resulting head color for HEAD-COLOR and BODY-COLOR."
@@ -839,12 +847,17 @@ result of `defhydra'."
       (cond ((< len 2)
              (error "Each head should have at least two items: %S" h))
             ((= len 2)
-             (setcdr (cdr h) `("" :cmd-name ,cmd-name)))
+             (setcdr (cdr h)
+                     (list
+                      (hydra-plist-get-default (cddr body) :hint "")
+                      :cmd-name cmd-name)))
             (t
              (let ((hint (cl-caddr h)))
                (unless (or (null hint)
                            (stringp hint))
-                 (setcdr (cdr h) (cons "" (cddr h))))
+                 (setcdr (cdr h) (cons
+                                  (hydra-plist-get-default (cddr body) :hint "")
+                                  (cddr h))))
                (setcdr (cddr h) `(:cmd-name ,cmd-name ,@(cl-cdddr h))))))))
   (let* ((keymap (copy-keymap hydra-base-map))
          (body-name (intern (format "%S/body" name)))