title: MDL add ripple effect when clicking 'mdl-navigation__link'
body: <p>I have such navigation:</p>
  
  <pre><code>&lt;nav class="mdl-navigation mdl-js-ripple-effect"&gt;
    &lt;a id="submenu" class="mdl-navigation__link" href="#"&gt;Link&lt;/a&gt;
    &lt;a class="mdl-navigation__link" href=""&gt;Link&lt;/a&gt;
    &lt;a class="mdl-navigation__link" href=""&gt;Link&lt;/a&gt;
    &lt;a class="mdl-navigation__link" href=""&gt;Link&lt;/a&gt;
  &lt;/nav&gt;
  </code></pre>
  
  <p>When <code>&lt;a&gt;</code> element clicked I want to add ripple effect on it with accent color.
  And then I want this element stayed selected. How can I do that?</p>