Tabs inside tabs

I needed to have a group of tabs inside a pill tab group, so i made this change.

It's not pretty but it works quite well.

If the user wants to do this, all that is needed is to include a "tabSet" attribute in the tab group ul, li and tab-content divs.
For each group of tabs, give tabSet a different value.

Example:
```html
<section class="tabs" tabSet='main'>
  <ul class="tab-nav">
    <li tabSet='main'><a href="#">Tab 1</a></li>
    <li tabSet='main' class="active"><a href="#">Tab 2</a></li>
    <li tabSet='main'><a href="#">Tab 3</a></li>
  </ul>
  <div tabSet='main' class="tab-content">
    <p>1</p>
  </div>
  <div tabSet='main' class="tab-content active">
    <p>2</p>
  </div>
  <div tabSet='main' class="tab-content">
    <p>3</p>
  </div>
</section>