Add code block pre class: `options.codeClass`.

If we want to use [google-code-prettify](http://google-code-prettify.googlecode.com/svn/trunk/README.html) easily, we need to custom code block pre classname.

Set `options.codeClass="prettyprint"` and get `<pre class="prettyprint">` html result is pretty good.

@chjj