Make number of spaces after code fence customizable

Fixes #219

CHANGES.md

@@ -187,6 +187,8 @@
         <kbd>C-c C-f</kbd>, <kbd>C-c C-b</kbd>, and <kbd>C-c C-u</kbd>
         now move between list items, when the point is in a list,
         and move between headings otherwise.
+    -   New customization option `markdown-spaces-after-code-fence` to control
+        the number of spaces inserted after a code fence (` ``` `).
 
 *   Improvements:
 

markdown-mode.el

@@ -4873,6 +4873,15 @@ if three backquotes inserted at the beginning of line."
   (setq markdown-gfm-used-languages
           (cons lang (remove lang markdown-gfm-used-languages))))
 
+(defcustom markdown-spaces-after-code-fence 1
+  "Number of space characters to insert after a code fence.
+\\<gfm-mode-map>\\[markdown-insert-gfm-code-block] inserts this many spaces between an
+opening code fence and an info string."
+  :group 'markdown
+  :type 'integer
+  :safe #'natnump
+  :package-version '(markdown-mode . "2.3"))
+
 (defun markdown-insert-gfm-code-block (&optional lang)
   "Insert GFM code block for language LANG.
 If LANG is nil, the language will be queried from user.  If a
@@ -4890,7 +4899,9 @@ automatically in order to have the correct markup."
                  'markdown-gfm-language-history))
              (quit "")))))
   (unless (string= lang "") (markdown-gfm-add-used-language lang))
-  (when (> (length lang) 0) (setq lang (concat " " 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)))
         (goto-char e)

tests/markdown-test.el

@@ -4956,6 +4956,11 @@ This includes preserving whitespace after the pipe."
    (should (equal (car (markdown-gfm-get-corpus)) "elisp"))
    (should (string-equal (buffer-string)
                          "line 1\n\n``` elisp\n\n```\n\nline 2\n")))
+  ;; Test ‘markdown-spaces-after-code-fence’.
+  (markdown-test-string-gfm ""
+    (let ((markdown-spaces-after-code-fence 0))
+      (markdown-insert-gfm-code-block "elisp")
+      (should (equal (buffer-string) "```elisp\n\n```"))))
   ;; Test with active region
   (markdown-test-string-gfm "line 1\nline 2\nline 3\n"
    (forward-line)