Refactor Asciidoctor PDF as a Prawn view

[mojavelinux]

Make use of the Prawn "view" API instead of extending Prawn to isolate the Asciidoctor PDF methods from Prawn core.

For more information, refer to https://github.com/prawnpdf/prawn/blob/master/lib/prawn/view.rb

[mojavelinux]

More information about Prawn::View can be found in the 1.3.0 release notes:

https://github.com/prawnpdf/prawn/wiki/CHANGELOG#added-the-prawnview-mixin-for-using-prawns-dsl-in-your-own-classes

[mojavelinux]

I didn't know Prawn very well when I started it, so we ended up with a monolithic subclass of Document. We really need to upgrade Prawn and migrate to a Prawn View. The first step is a simple demonstration of how that might look. Don't worry about all the node handlers to start with. We need a vision.

[mojavelinux]

The view is now documented in the manual. See https://github.com/prawnpdf/prawn/blob/master/manual/basic_concepts/view.rb