Rewrite some parts of extension, adding a Page class

[Owen T. Heisler]

The extension needs to have some parts rewritten. Specifically, a separate Page class, unique to this extension, needs to be added to represent each separate Page in the resulting output, rather than having this extension's code spread around extending the other stock Asciidoctor classes.

This should simplify the code significantly, making it easier to understand, and will make it much easier to resolve open issues (even resolve some of them without any work on them specifically).

This issue needs to be completed before most of the other open issues can be worked on.

[Owen T. Heisler]

See also my sponsor page.