Add M-RET binding for terminal

* lisp/org.el (org-mode-map): Add M-RET binding.

Reported-by: Allen Li <vianchielfaura@gmail.com>
<http://lists.gnu.org/archive/html/emacs-orgmode/2017-09/msg00386.html>

lisp/org.el

@@ -19482,6 +19482,7 @@ COMMANDS is a list of alternating OLDDEF NEWDEF command names."
 (org-defkey org-mode-map [(shift return)]   'org-table-copy-down)
 (org-defkey org-mode-map [(meta shift return)] 'org-insert-todo-heading)
 (org-defkey org-mode-map [(meta return)]       'org-meta-return)
+(org-defkey org-mode-map (kbd "M-RET") #'org-meta-return)
 
 ;; Cursor keys with modifiers
 (org-defkey org-mode-map [(meta left)]  'org-metaleft)