Enable callback usage in Prettyprint.

This enables the possibility to use callbacks with PrettyPrint.
It is fairly easy: place in your template.values file, in whatever section you need to use the callback, a variable named 'prettyPrintCallbackName', and assign an UNIQUE name you will be using for that callback.
Then, in the corrispective JS file of your template, do something like window['your_callback_name'] = function() { console.log ('Yay!'); }. Voila, you're done.
This was done to circumvent an issue with the current NERDZ architecture: there is not a way to alter how BBCodes are displayed. My intention was to edit the output of the [code] tag, but without editing the main codebase this is impossible without using an hack like this.
This commit does not break anything - any template not specifying this variable will not be affected.
Merge pull request #56 from Robertof/patch-prettyprint

Enable callback usage in PrettyPrint.