ox-md.el: Fix export of irc and mailto links

* lisp/ox-md.el (org-md-link): Add mailto and irc to links that pass
their types through to markdown export

See: https://github.com/larstvei/ox-gfm/issues/17  for an external
bug report

TINYCHANGE

lisp/ox-md.el

@@ -411,7 +411,7 @@ a communication channel."
      (t (let* ((raw-path (org-element-property :path link))
 	       (path
 		(cond
-		 ((member type '("http" "https" "ftp"))
+		 ((member type '("http" "https" "ftp" "mailto" "irc"))
 		  (concat type ":" raw-path))
 		 ((string= type "file")
 		  (org-export-file-uri (funcall link-org-files-as-md raw-path)))