Plugin system for different publishers

[nifey]

Right now we have hardcoded some HTML transformation like removing epub:type, converting image tag to img tag, etc.
Different epub creators have different structure, like title page, TOC, about page, etc.

It would be nice to have some hooks similar to plugins of Ebooklib to allow doing some extra modifications specific to an epub creator. Eg: Books from project gutenberg have a notice at the beginning and end. This hook can allow moving the notice to a different chapter.