title: add javascript code in knitr
body: <p>How do I add javascript code to a <code>Rmd</code>?</p>
  
  <p>I'm using the following chunk options:</p>
  
  <pre><code>```{r engine = 'js', results='asis', echo = T, highlight = T}
  var i = 1 + 1;
  ``` 
  </code></pre>
  
  <p>And this produces a <code>.md</code> with:</p>
  
  <pre><code>&lt;script type="text/javascript"&gt;
  var i = 1 + 1;
  &lt;/script&gt;
  </code></pre>
  
  <p>I would like to have something like:</p>
  
  <pre><code>```js
  var i = 1 + 1;
  ```
  
  &lt;script type="text/javascript"&gt;
  var i = 1 + 1;
  &lt;/script&gt;
  </code></pre>
  
  <p>In other words, I don't need the code to be evaluated when knitting, I just need that highlighted code appears in the output.</p>