use GFM extensions for markdown export, closes #638

src/services/export/md.js

@@ -0,0 +1,19 @@
+"use strict";
+
+const TurndownService = require('turndown');
+const turndownPluginGfm = require('turndown-plugin-gfm');
+
+let instance = null;
+
+function toMarkdown(content) {
+    if (instance === null) {
+        instance = new TurndownService();
+        instance.use(turndownPluginGfm.gfm);
+    }
+
+    return instance.turndown(content);
+}
+
+module.exports = {
+    toMarkdown
+};