Feature standalone rules

This pull requests extracts the Replacer logic from the listener and the database driven rule models. 

With this changes it will be possible to load different rules into the replacer which can do anything to the dom document.

So there is no need to create another dom representation of the current document if other changes should be applied dynamically as well.

To summarize the changes:
 * Introduce a `RuleInterface` and extract the css class replacer logic to `ClassAttributeRule`
 * Introduce a `DomManipulator` which can be used without the `Listener`
 * Provide a `loadCssClassReplacerRules` hook so that other rules can be loaded.

Wenn es nach mir geht, würd ich den EventDispatcher nutzen. Das Event bekommt die Methoden `addRule` und `addRules` mit.

Wenn wir beim Hook bleiben, ist per Referenz in Ordnung. Hatte mich daran orientiert, was Contao gewöhnlich macht.