[vertical-markdown] Changed up pre/post process algorithm

Previously, we'd let the markdown processor take "---" and create an
"<hr/>" out of it.. and then we'd go to work with what was left.

Now, we explicitly seek out "---" and switch it to something the
markdown processor won't touch and doesn't care about-- in the
preprocessing stage: a boring old <div/>.

This is nicer.. but it's happening because I'm planning to use "***" as
the separator for vertical slides, and both --- and *** will end up as
"<hr/>"s.

If that happens-- I won't know which is which in the postproces step.
[vertical-markdown] Refactored pre/postprocess toward OO
[vertical-markdown] Started supporting '***' for vertical slides. Incomplete.
[vertical-markdown] Broke Pre/Post process specs out into distinct files
[vertical-markdown] Created Vertical Start+End.
[vertical-markdown] Supported back-to-back verticals
[vertical-markdown] Enhanced PostProcessor coverage
[vertical-markdown] Added additional feature coverage
Added support for vertical slides in Markdown

Merge branch 'vertical-markdown' into upcoming

* vertical-markdown:
  [vertical-markdown] Added additional feature coverage
  [vertical-markdown] Enhanced PostProcessor coverage
  [vertical-markdown] Supported back-to-back verticals
  [vertical-markdown] Created Vertical Start+End.
  [vertical-markdown] Broke Pre/Post process specs out into distinct files
  [vertical-markdown] Started supporting '***' for vertical slides. Incomplete.
  [vertical-markdown] Refactored pre/postprocess toward OO
  [vertical-markdown] Changed up pre/post process algorithm
CHANGELOG and version