Use stable anchors in table of contents.

Github-flavoured markdown renderers automatically add anchors to
headings, this removes the org-generated ones (which change every time
you export) in favour of the standard.

ox-gfm.el

@@ -1,6 +1,8 @@
 ;;; ox-gfm.el --- Github Flavored Markdown Back-End for Org Export Engine
 
 ;; Copyright (C) 2014-2017 Lars Tveito
+;; Copyright (C) 2012-2018 Free Software Foundation, Inc.
+;; Copyright (C) 2018 Aaron Muir Hamilton
 
 ;; Author: Lars Tveito
 ;; Keywords: org, wp, markdown, github
@@ -53,7 +55,8 @@
             (lambda (a s v b)
               (if a (org-gfm-export-to-markdown t s v)
                 (org-open-file (org-gfm-export-to-markdown nil s v)))))))
-  :translate-alist '((inner-template . org-gfm-inner-template)
+  :translate-alist '((headline . org-gfm-headline)
+                     (inner-template . org-gfm-inner-template)
                      (paragraph . org-gfm-paragraph)
                      (strike-through . org-gfm-strike-through)
                      (src-block . org-gfm-src-block)
@@ -65,6 +68,47 @@
 
 ;;; Transcode Functions
 
+;;;; Headline
+(defun org-gfm-headline (headline contents info)
+  "Transcode HEADLINE element into Markdown format.
+CONTENTS is the headline contents.  INFO is a plist used as
+a communication channel."
+  (unless (org-element-property :footnote-section-p headline)
+    (let* ((level (org-export-get-relative-level headline info))
+	   (title (org-export-data (org-element-property :title headline) info))
+	   (todo (and (plist-get info :with-todo-keywords)
+		      (let ((todo (org-element-property :todo-keyword
+							headline)))
+			(and todo (concat (org-export-data todo info) " ")))))
+	   (tags (and (plist-get info :with-tags)
+		      (let ((tag-list (org-export-get-tags headline info)))
+			(and tag-list
+			     (format "     :%s:"
+				     (mapconcat 'identity tag-list ":"))))))
+	   (priority
+	    (and (plist-get info :with-priority)
+		 (let ((char (org-element-property :priority headline)))
+		   (and char (format "[#%c] " char)))))
+	   ;; Headline text without tags.
+	   (heading (concat todo priority title))
+	   (style (plist-get info :md-headline-style)))
+      (cond
+       ;; Cannot create a headline.  Fall-back to a list.
+       ((or (org-export-low-level-p headline info)
+	    (not (memq style '(atx setext)))
+	    (and (eq style 'atx) (> level 6))
+	    (and (eq style 'setext) (> level 2)))
+	(let ((bullet
+	       (if (not (org-export-numbered-headline-p headline info)) "-"
+		 (concat (number-to-string
+			  (car (last (org-export-get-headline-number
+				      headline info))))
+			 "."))))
+	  (concat bullet (make-string (- 4 (length bullet)) ?\s) heading tags "\n\n"
+		  (and contents (replace-regexp-in-string "^" "    " contents)))))
+       (t
+	(concat (org-md--headline-title style level heading tags) contents))))))
+
 ;;;; Paragraph
 
 (defun org-gfm-paragraph (paragraph contents info)
@@ -236,8 +280,7 @@ plist used as a communication channel."
                  (org-export-get-alt-title headline info) info))
          (level (1- (org-element-property :level headline)))
          (indent (concat (make-string (* level 2) ? )))
-         (anchor (or (org-element-property :CUSTOM_ID headline)
+         (anchor (replace-regexp-in-string "[^a-z\-]" "" (downcase (replace-regexp-in-string "[ ]" "-" title)))))
-                     (org-export-get-reference headline info))))
     (concat indent "- [" title "]" "(#" anchor ")")))