Fix invalid nested ul

[toshimaru]

Issue #25

Changes

• Add last_ul_used arguments to fix invalid nested <ul> tag
• Add more complex toc test
• Add kramdown list parser test for reference

Parser logic overview

<h6>h6</h6>
<h5>h5</h5>
<h4>h4</h4>
<h3>h3</h3>
<h2>h2</h2>
<h1>h1</h1>

This headings generate following TOC.

<ul class="section-nav">
<li class="toc-entry toc-h6"><a href="#h6">h6</a></li>
<li class="toc-entry toc-h5"><a href="#h5">h5</a></li>
<li class="toc-entry toc-h4"><a href="#h4">h4</a></li>
<li class="toc-entry toc-h3"><a href="#h3">h3</a></li>
<li class="toc-entry toc-h2"><a href="#h2">h2</a></li>
<li class="toc-entry toc-h1"><a href="#h1">h1</a></li>
</ul>

---

<h1>h1</h1>
<h3>h3</h3>
<h2>h2</h2>
<h4>h4</h4>
<h5>h5</h5>

This headings generate following TOC.

<ul class="section-nav">
<li class="toc-entry toc-h1">
<a href="#h1">h1</a>
<ul>
<li class="toc-entry toc-h3"><a href="#h3">h3</a></li>
<li class="toc-entry toc-h2">
<a href="#h2">h2</a>
<ul>
<li class="toc-entry toc-h4">
<a href="#h4">h4</a>
<ul>
<li class="toc-entry toc-h5"><a href="#h5">h5</a></li>
</ul>
</li>
</ul>
</li>
</ul>
</li>
</ul>