<ul> nested inside <ol> needs three space indent, not two

[snarfed]

First off, thank you for maintaining html2text, it's great!

Right now, list items are always indented two spaces per enclosing list. This is generally right, but doesn't work for <ul>s nested inside <ol>s. Those need three spaces (at least) instead. Details here in the Common Mark spec.

I have a PR ready for this, I'll submit it in a minute.

For example, this HTML:

<ol>
  <li>ordered</li>
  <ul>
    <li>unordered</li>
  </ul>
</ol>

needs to convert to this, with five spaces before * unordered instead of four:

  1. ordered
     * unordered
  3. end

Tested against HEAD with multiple Python 3 versions.