add afterrender hook to View class

Would be nice to have an `afterrender` hook for the templating. This way we could add own functions to parse the generated templates and modify it slightly before it goes into the `/tmp` cache as pre-compiled final template.

A use case for that could be something like my [draft for an css/js asset manager](https://gist.github.com/ikkez/ef2cfaa3009c439ea484). It wouldn't be possible without such an event/hook, because there is currently no other way to modify the content of a parent template from the inside of a sub-template. With that after-renderer we can finalize some modifications of the template we might have started in the pre-compile process (adding additional marker or php code).

so far. hope you like it.