only run customwmd when where's markdown

lms/static/coffee/src/customwmd.coffee

 $ ->
+
+  if Markdown?
+    mathRenderer = new MathJaxDelayRenderer()
+    removeMath = (text) -> text
+
+    replaceMath = (text) -> text
+
+    updateMathJax = ->
+      console.log "updating"
+      #mathRenderer.render
+      #  element: $("#wmd-preview")
+      MathJax.Hub.Queue(["Typeset", MathJax.Hub, "wmd-preview"])
+
+
     converter = Markdown.getSanitizingConverter()
     editor = new Markdown.Editor(converter)
-  #converter.hooks.chain "preConversion", removeMath
+    converter.hooks.chain "preConversion", removeMath
+    converter.hooks.chain "postConversion", replaceMath
+    editor.hooks.chain "onPreviewRefresh", updateMathJax
     editor.run()