How can I add classes to the `<a>` or `<li>` tags of the main menu to signify which level they are in the menu hierarchy (e.g. "level-1", "level-2", etc.)? The code below shows the first link has 3 sublinks (dropdown):

```
<ul class="menu">
  <li class="first expanded">
    <a class="level-1" href="#" title="">Top link</a>
    <ul class="menu">
      <li class="first leaf"><a class="level-2" href="#">Dropdown 1</a></li>
      <li class="leaf"><a class="level-2" href="#">Dropdown 2</a></li>
      <li class="last leaf"><a class="level-2" href="#">Dropdown 3</a></li>
    </ul>
  </li>
  <li class="last leaf">
    <a class="level-1" href="#" title="">Another top link</a>
  </li>
</ul>

```