Add support for comments with CDATA

Some comments have CDATA in them. The one I saw seemed to be pasted from microsoft word. This results in an XML file that will not be imported but have an error. A simple solution is remove the closing CDATA tag in the filter function like so:

function disqus_migrate_filter($text) {
  $text = str_replace("\r", '', $text);
  return str_replace("]]>", '', filter_dom_serialize(filter_dom_load($text)));
}

There may be a better solution.