[RichString] Add markdown support Supporting markdown requires the 'Markdown' python packages, and since it widely available on the target platforms adding it as a strong requirements will not be a big constraint. Closes #3814302

<div id="ajaxroot">
  <div class="ajaxHtmlHead">
    <script src="http://foo.js" type="text/javascript"> </script>
    <link rel="stylesheet" type="text/css" media="all" href="qunit.css" />
  </div>
  <h1>Hello</h1>
</div>