Add highlight test

Re #145 @azaroth42, this is my output from `highlight.md`:

``` html
<h2 id="backticks">Backticks</h2>

<pre><code class="language-html">&lt;a href="#"&gt;Highlight&lt;/a&gt;
</code></pre>

<h2 id="tildes">Tildes</h2>

<pre><code class="language-html">&lt;a href="#"&gt;Highlight&lt;/a&gt;
</code></pre>

<h2 id="highlight">Highlight</h2>

<div class="highlight">
  <pre><code class="html"><span class="nt">&lt;a</span> <span class="na">href=</span><span class="s">&quot;#&quot;</span><span class="nt">&gt;</span>Highlight<span class="nt">&lt;/a&gt;</span></code></pre>
</div>
```