Support nested list

[yhatt]

jsx-slack had allowed lists with only one level. And now, we will get the support of nested <ul> and <ol> by improved building process of mimicked text. It would get better HTML compatibility.

<Block>
  <section>
    <p>
      <strong>Nested list</strong><br />
      (from <a href="https://developer.mozilla.org/en-US/docs/Web/HTML/Element/ul#Nesting_list">https://developer.mozilla.org/en-US/docs/Web/HTML/Element/ul#Nesting_list</a>)
    </p>
  </section>
  <hr />
  <section>
    <ul>
      <li>first item</li>
      <li>second item
        <ul>
          <li>second item first subitem</li>
          <li>second item second subitem
            <ul>
            <li>second item second subitem first sub-subitem</li>
            <li>second item second subitem second sub-subitem</li>
            <li>second item second subitem third sub-subitem</li>
            </ul>
          </li>
          <li>second item third subitem</li>
        </ul>
      </li>
      <li>third item</li>
    </ul>
  </section>
</Block>

HTML	Slack