title: Set data attribute in html element when slide is active
body: <p>hy,</p>
  
  <p>im using FlexSlider to my web site (Slider w/thumbnail controlNav pattern), I made some modification of style of thumbnail, my question is how can i set data-thumb attribute in li element when the slider is active, in css they juste change the opacity of img i want to change the src image this the class css</p>
  
  <pre><code>  .flex-control-thumbs .flex-active {
    opacity: 1;
    cursor: default;
  }
  </code></pre>
  
  <p>html code</p>
  
  <pre><code>&lt;div class="flexslider"&gt;
                &lt;ul class="slides"&gt;
  
                  &lt;li data-thumb="assets/img/nav-slide.png"&gt;
                    &lt;img src="assets/img/sld2.jpg" /&gt;
                  &lt;/li&gt;
  
                &lt;/ul&gt;
              &lt;/div&gt;
  </code></pre>
  
  <p>there is any way to set data-thumb using css3, i want to be for example like this when the slide thumbnail in mode active</p>
  
  <pre><code> &lt;div class="flexslider"&gt;
                &lt;ul class="slides"&gt;
  
                  &lt;li data-thumb="assets/img/active-nav-slide.png"&gt;
                    &lt;img src="assets/img/sld2.jpg" /&gt;
                  &lt;/li&gt;
  
                &lt;/ul&gt;
              &lt;/div&gt;
  </code></pre>