Propertize GFM code blocks upon insertion

See GH-251.

markdown-mode.el

@@ -5131,13 +5131,14 @@ automatically in order to have the correct markup."
                  (markdown-gfm-get-corpus)
                  nil 'confirm (car markdown-gfm-used-languages)
                  'markdown-gfm-language-history))
-             (quit "")))))
+             (quit "")))
+         current-prefix-arg))
   (unless (string= lang "") (markdown-gfm-add-used-language lang))
   (when (> (length lang) 0)
     (setq lang (concat (make-string markdown-spaces-after-code-fence ?\s)
                        lang)))
   (if (markdown-use-region-p)
-      (let* ((b (region-beginning)) (e (region-end))
+      (let* ((b (region-beginning)) (e (region-end)) end
              (indent (progn (goto-char b) (current-indentation))))
         (goto-char e)
         ;; if we're on a blank line, don't newline, otherwise the ```
@@ -5147,6 +5148,7 @@ automatically in order to have the correct markup."
         (indent-to indent)
         (insert "```")
         (markdown-ensure-blank-line-after)
+        (setq end (point))
         (goto-char b)
         ;; if we're on a blank line, insert the quotes here, otherwise
         ;; add a new line first
@@ -5155,17 +5157,20 @@ automatically in order to have the correct markup."
           (forward-line -1))
         (markdown-ensure-blank-line-before)
         (indent-to indent)
-        (insert "```" lang))
-    (let ((indent (current-indentation)))
+        (insert "```" lang)
+        (markdown-syntax-propertize-fenced-block-constructs (point-at-bol) end))
+    (let ((indent (current-indentation)) start)
       (delete-horizontal-space :backward-only)
       (markdown-ensure-blank-line-before)
       (indent-to indent)
+      (setq start (point))
       (insert "```" lang "\n")
       (indent-to indent)
       (insert ?\n)
       (indent-to indent)
       (insert "```")
-      (markdown-ensure-blank-line-after))
+      (markdown-ensure-blank-line-after)
+      (markdown-syntax-propertize-fenced-block-constructs start (point)))
     (end-of-line 0)))
 
 (defun markdown-code-block-lang (&optional pos-prop)