Add support for Persistent Transformations

There are some use cases where it would be interesting to be able to save the result of the execution of a Transformation in the original content.

For example, inserting a signature, see http://markmail.org/message/batosctqycyvk2a5

Note that currently it can be achieved with a listener but it has drawbacks:

• it's not efficient in term of performances and memory since that listener will always be present in memory and always execute whenever a page is saved
• is not natural to write and a Persistent Transformation seems a more natural concept.