title: Add a CSS class to single code chunks in RMarkdown
body: <p>Is it possible to add a CSS class to a certain code chunk?</p>
  
  <p>Assume the following file:</p>
  
  <pre><code>---
  title: "Untitled"
  output: html_document
  ---
  
  
  ```{r cars}
  summary(cars)
  ```
  </code></pre>
  
  <p>I want to give the chunk labeled 'cars' a certain CSS class, e.g. <code>.myClass</code>.
  Is there any possibility like</p>
  
  <pre><code>```{r cars} {.myClass}
  summary(cars)
  ```
  </code></pre>
  
  <p>or so? I am aware of hacks like wrapping the whole chunk in another <code>&lt;div&gt;</code>. I am interested in a straight forward solution.</p>