title: How can I select all options of multi-select select box on click?
body: <p>This is my HTML</p>
  
  <pre><code>&lt;select name="countries" id="countries" MULTIPLE size="8"&gt;
     &lt;option value="UK"&gt;UK&lt;/option&gt;
     &lt;option value="US"&gt;US&lt;/option&gt;
     &lt;option value="Canada"&gt;Canada&lt;/option&gt;
     &lt;option value="France"&gt;France&lt;/option&gt;
     &lt;option value="India"&gt;India&lt;/option&gt;
     &lt;option value="China"&gt;China&lt;/option&gt;
  &lt;/select&gt;
  &lt;br /&gt;
  &lt;input type="button" id="select_all" name="select_all" value="Select All"&gt;
  </code></pre>
  
  <p>When user click on 'Select All' button, I want all the options in the select box to be selected</p>
  
  <pre><code>$('#select_all').click( function() {
      // ?
  });
  </code></pre>