Use textContent instead of innerHTML to prevent encoding of certain entities in Markdown code blocks.

This is to fix an issue I encountered with commit 78b9bb4 and markdown processing. The following snippet will be rendered in reveal with the greater than transformed to a '&amp;gt;'

```
<section data-markdown>
## A Markdown slide

	square = (x) -> x*x

End code block
</section>
```

Markdown.pl will output the following HTML for this snippet:
```
<h2>A Markdown slide</h2>

<pre><code>square = (x) -&gt; x*x
</code></pre>

<p>End code block</p>
```

The [showdown preview](http://softwaremaniacs.org/playground/showdown-highlight/) does not output an entity:

```
<h2>A Markdown slide</h2>

<pre><code>square = (x) -> x*x
</code></pre>

<p>End code block</p></div>
```