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.

I am not sure if this is really the clean way to go.
But using content did not allow me to restructure the tree.
Because in the end in asciidoctor the section owns its subsections, thus forming a parent-child-relationship.
For reveal.js a sibling relationship is required, i.e. [section, subsection, subsection, ...]