title: how show file-like icon for leaf nodes in jstree 3
body: <p>I see that jstree 3 put class jstree-leaf for li element of leaf nodes(so it recognize them automatically). How can I configure types plugin to show <code>jstree-icon jstree-file</code> icon for them. is there anything like this for it:</p>
  
  <pre><code>'types' : {
              'file' : {
                  'icon': 'jstree-icon jstree-file'
              }
          },
  </code></pre>