Improve markdown support. (#817)

1. Support `marked`, you can using it instead. #811
2. Auto trim the indentation, so it can be indent like regular HTML.
   #812

examples/markdown/index.html

@@ -182,6 +182,7 @@ toggle the CSS style:</p>
 <script type="text/javascript" src="../../extras/highlight/highlight.pack.js"></script>
 <script type="text/javascript" src="../../extras/mermaid/mermaid.min.js"></script>
 <script type="text/javascript" src="../../extras/markdown/markdown.js"></script>
+<!-- <script src="https://cdn.jsdelivr.net/npm/marked/marked.min.js"></script> -->
 
 <!--
     To make all described above really work, you need to include impress.js in the page.