Add route to ShortcodeableController

Routes to controllers now need to be explicitly defined: https://docs.silverstripe.org/en/3.2/changelogs/3.2.0/#api-removed-url-routing-by-controller-name

_config/config.yml

@@ -3,4 +3,7 @@ LeftAndMain:
     - 'shortcodable/javascript/editor_plugin.js'
     - 'shortcodable/javascript/shortcodable.js'
   extra_requirements_css:
-    - 'shortcodable/css/shortcodable.css'
+    - 'shortcodable/css/shortcodable.css'
+Director:
+  rules:
+    ShortcodableController: ShortcodableController