Yield the humanized id of the headers

Hello,

Using the `header` callback, it's now possible to get access to the humanized generated id to easily keep tracking of the tree of headers or simply handle the duplicate values easily.

Since the `HTML_TOC` and `HTML` objects both have this callback, it's advisable to define a module and mix it in these objects to avoid code duplication.

It should resolve #310 since it's now straightforward to customize the anchors (replacing the "&" with "and" for instance).

Have a nice day.