Issue title: HTML Stripper
 Issue body: So, the syntax highlighting for html doesn't really do what I'd expect it to.  I have a block of html and want to highlight it.  

``` html
<pre class="language-markup">
  <code class="language-markup">

    <div class="flexslider">

      <ul class="slides">

        <li>
          <img src="img/1x1.gif" data-original="img/slideshow/1080/grey01.jpg">
        </li>

        <li>
          <img src="img/1x1.gif" data-original="img/slideshow/1080/grey02.jpg">
        </li>

        <li>
          <img src="img/1x1.gif" data-original="img/slideshow/1080/grey03.jpg">
        </li>

        <li>
          <img src="img/1x1.gif" data-original="img/slideshow/1080/grey14.jpg">
        </li>

        <li>
          <img src="img/1x1.gif" data-original="img/slideshow/1080/grey15.jpg">
        </li>

        <li>
          <img src="img/1x1.gif" data-original="img/slideshow/1080/grey16.jpg">
        </li>

      </ul>

    </div>

  </code>
</pre>
```

Read the known issue [http://prismjs.com/faq.html#if-pre-existing-html-is-stripped-off-how-can-i-highlight](http://prismjs.com/faq.html#if-pre-existing-html-is-stripped-off-how-can-i-highlight) + reviewed how implemented on [prismjs.com](http://prismjs.com/index.html) so it's obsss possible

What am I missing?

 Comments: 
 Comment 0: Not sure what you mean. The FAQ item you link to is not relevant to the fact that pre-existing HTML in code blocks will be stripped out before highlighted. That’s a limitation that's mentioned in Prism's homepage.
What you need to do to highlight this code is to encode `<` as `&lt;` (at least), so that it’s not parsed as HTML by the browser.
Btw you don't need to use the class `language-markup` twice, once is enough.

Also (off topic), are you sure spacer gifs are a good idea in this flexslider? You could just remove the `<img>` altogether and apply the background to the `<li>` or something.

 Comment 1: Thanks Lea, really appreciate the work you've put into this project!  Yeah, that Flexslider markup isn't production.  The actual version is lazyload + picturefill, so it's a hot mess...  I might try optimizing Flexslider using bg images next, though solution has to work w/ cms user uploaded content, so not sure how the templating would work.  Maybe inline styles in the template?

Anyways, for anyone else looking syntax highlight HTML, just need to encode.  I used [htmlentities.net/](http://htmlentities.net/), worked great! 

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