MD035: Add "slides" option to allow reveal.js vertical/horizontal slide feature (multiple lengths)

[RonnyPfannschmidt]

context: when creating reveal.js slides in markdown, its possible to use longer hr for splitting vertical/horizontal slides

it would be nice if it was possible to configure a slide split mode that allowed 2 specified and consistent lengths of the hr markers

example

## My  presentation
# intro
slide 1.1
----

slide 1.2

-----

slide 2.1

---
slide 2.2