ox-md: Export TODO keywords and priority cookies

* lisp/ox-md.el (org-md-headline): Export TODO keywords and priority
  cookies.

Reported-by: Ganesh Krishnan <gkrishnan.gt@gmail.com>
<http://lists.gnu.org/archive/html/emacs-orgmode/2017-10/msg00171.html>

lisp/ox-md.el

@@ -240,7 +240,7 @@ a communication channel."
 		    (format "<a id=\"%s\"></a>"
 			    (or (org-element-property :CUSTOM_ID headline)
 				(org-export-get-reference headline info))))))
-	  (concat (org-md--headline-title style level title anchor tags)
+	  (concat (org-md--headline-title style level heading anchor tags)
 		  contents)))))))