jekyll-toc: Invalid HTML

Hey, there.

So, in Bootstrap v4-dev we switched to your plugin.

I notice that the generated HTML is invalid.

See for example https://github.com/twbs/bootstrap/blob/v4-dev/docs/4.0/about/brand.md

The HTML we get is the following, which is invalid:

            <ul class="section-nav">
<ul>
<li class="toc-entry toc-h2"><a href="#contents">Contents</a></li>
<li class="toc-entry toc-h2"><a href="#mark-and-logo">Mark and logo</a></li>
</ul>
<li class="toc-entry toc-h1"><a href="#bootstrap">Bootstrap</a></li>
<li class="toc-entry toc-h1"><a href="#bootstrap-1">Bootstrap</a>
<ul>
<li class="toc-entry toc-h2"><a href="#download-mark">Download mark</a></li>
<li class="toc-entry toc-h2"><a href="#name">Name</a>
<ul>
<li class="toc-entry toc-h3"><a href="#bootstrap-2">Bootstrap</a></li>
<li class="toc-entry toc-h3"><a href="#bootstrap-3">BootStrap</a></li>
<li class="toc-entry toc-h3"><a href="#twitter-bootstrap">Twitter Bootstrap</a></li>
</ul>
</li>
<li class="toc-entry toc-h2"><a href="#colors">Colors</a></li>
</ul>
</li>
</ul>

/CC @mdo @johann-S