Add output transformers

[vittoriom]

Similar to value transformers, introduce "output transformers", that transform values of one type to values of the same type but apply some logic on top (for example resize images, sanitize strings, etc). These transformers can be chained to CacheLevels and will affect the data that will be returned by get requests, but not the data that will be saved on set requests, as it happens for normal value transformers.