Add the options for themes to manipulate the post content

[Chumper]

I have created a new theme so far and wanted to integrate prettify for the codepreview.
In order to do that every <pre><code> needs to be changed to <pre class="prettyprint linenums"><code>.

So i would suggest an option for the theme to do a str_replace or something similar to the parsed content.

This could be done directly in the view but i think that shoul belong somewhere else.