Fix issue #1638: support dynamic loading marked.sj with fallback for v<6

In v6 jupyter notebook components/marked/lib/marked no longer exists. The extension silently falls over.
Hack around puts local copy of marked.min from cdnjs.com into the python-markdown directory.

require 'marked' on two paths,
first expecting marked.min.j in the same dir,
second expecting marked in the v6 compenents/ location.

Insert ERROR message in user page if neither is found.

Author: johnjbarton

src/jupyter_contrib_nbextensions/nbextensions/python-markdown/main.js

@@ -13,12 +13,29 @@ define([
     'require',
     'notebook/js/cell',
     'base/js/security',
-    'components/marked/lib/marked',
+    // marked loaded dynamically for v5 vs v6
     'base/js/events',
     'notebook/js/textcell'
-], function(IPython, $, requirejs, cell, security, marked, events, textcell) {
+], function(IPython, $, requirejs, cell, security, events, textcell) {
     "use strict";
 
+    var marked = null;
+    requirejs('nbextensions/python-markdown/marked.min', 
+        function(marked_local) {
+            marked = marked_local.marked;
+        }, 
+        function(e_ignored) {
+            // fall back to components for v<6.
+            requirejs('components/marked/lib/marked', 
+                function(marked_compat) {
+                    marked = marked_compat;
+                }, 
+                function(error) {
+                    marked = function() {
+                        return  "ERROR: marked JS library required but not found";
+                    }
+                })
+        });
     /*
      * Find Python expression enclosed in {{ }}, execute and add to text as
      * <span> tags. The actual content gets filled in later by a callback.