Add support for vertical slides

Adds support for vertical slides.
Subsections will be rendered "below" the owning sections.
The paragraphs of the section define the top slide.

Sure thing! It would be even better if the person who knows the API (me looks in mirror) would actually document said API ;)

For reference, here's a very rough look at the API: http://www.rubydoc.info/github/asciidoctor/asciidoctor/Asciidoctor/AbstractNode