Added Extra transformer feature and fixed tests

Hello,

I fixed the tests on the bundle but I also add a new way to add basic transformers on text. That's because Wordpress parse freely anywere in his code the content of a blog post.

For example, Wordpress parse the content to add videos as iframe instead of links.

So I:

* Added a transformer registry
* Added new configuration to be able to disable non desired transformers
* Added new tests related to the new feature
* Fixed old tests (that should be improved)
* Fixed some CS