This adds a bunch of tests to better exercise the HTML rendering and to
be able to track any changes in its behavior.
This includes a new `check_all_main_files` to more conveniently check
the HTML content of every chapter in a book.
rel=edit lets a page indicate that the linked resource can be used to
edit the page. It is defined at https://microformats.org/wiki/rel-edit.
This can then be parsed by tools like the Universal Edit Button and
custom bookmarklets to open the edit page corresponding with a website.
