title: pandoc creates html file that does not format code blocks
body: <p>I am using pandoc to create a html file from an r markdown file, however the code in the html file is not highlighted or formatted, where the html file created from knithtml does show formatting. the pandoc command I am using is:</p>
  
  <pre><code>system('pandoc -s -t slidy mdown.md -o mdown.html')
  </code></pre>
  
  <p>Am I perhaps missing a flag option here?</p>