Fix double-escaping of # and ![ in Markdown export

* lisp/ox-md.el (org-md-plain-text): Fix the order of replacements in
  plain text.

<http://lists.gnu.org/r/emacs-orgmode/2017-12/msg00460.html>

lisp/ox-md.el

@@ -500,14 +500,15 @@ TEXT is the string to transcode.  INFO is a plist holding
 contextual information."
   (when (plist-get info :with-smart-quotes)
     (setq text (org-export-activate-smart-quotes text :html info)))
+  ;; The below series of replacements in `text' is order sensitive.
+  ;; Protect `, *, _, and \
+  (setq text (replace-regexp-in-string "[`*_\\]" "\\\\\\&" text))
   ;; Protect ambiguous #.  This will protect # at the beginning of
   ;; a line, but not at the beginning of a paragraph.  See
   ;; `org-md-paragraph'.
   (setq text (replace-regexp-in-string "\n#" "\n\\\\#" text))
   ;; Protect ambiguous !
   (setq text (replace-regexp-in-string "\\(!\\)\\[" "\\\\!" text nil nil 1))
-  ;; Protect `, *, _ and \
-  (setq text (replace-regexp-in-string "[`*_\\]" "\\\\\\&" text))
   ;; Handle special strings, if required.
   (when (plist-get info :with-special-strings)
     (setq text (org-html-convert-special-strings text)))