Support multi-level <ul> lists

[spekary]

It appears if you do this:

<ul>
  <li>Coffee</li>
  <li>Tea
    <ul>
      <li>Black tea</li>
      <li>Green tea</li>
    </ul>
  </li>
  <li>Milk</li>
</ul>

Html2Text will flatten the list. Just a request to support multiple level lists by adding additional tabs in front of each item per level.