title: In middleman, how do set variables?
body: <p>I've started experimenting with middleman and ruby.</p>
  
  <p>The sample layout has this string: </p>
  
  <pre><code>&lt;body class="&lt;%= page_classes %&gt;
  </code></pre>
  
  <p>On any given page, how do I set the 'page_class'?</p>