Nav tree

Nav with tree structure.
Example
<!-- Source -->
<ul class="nav nav-tree nav-x-0 nav-y-sm flex-column">
  <li class="nav-item"><a class="nav-link" href="#">Item 1</a></li>
  <li class="nav-item show"><a class="nav-link" href="#">Item 2</a>
    <ul class="nav nav-tree nav-x-0 nav-y-sm font-size-2 pl-3 flex-column">
      <li class="nav-item"><a class="nav-link" href="#">Sub-item 1</a></li>
      <li class="nav-item show"><a class="nav-link active" href="#">Sub-item 2</a>
        <ul class="nav nav-tree nav-x-0 nav-y-sm font-size-1 pl-3 flex-column">
          <li class="nav-item"><a class="nav-link" href="#">Sub-item 1</a></li>
          <li class="nav-item"><a class="nav-link active" href="#">Sub-item 2</a></li>
          <li class="nav-item"><a class="nav-link" href="#">Sub-item 3</a></li>
        </ul>
      </li>
      <li class="nav-item"><a class="nav-link" href="#">Sub-item 3</a></li>
    </ul>
  </li>
  <li class="nav-item"><a class="nav-link" href="#">Item 3</a></li>
</ul>