Moved location of require 'json' to central location

Bringing in json causes Hash to get a #to_json, and this is expected
later on in an erb file.

Might as well do it here so that it's in a central place.
[separate-triple-dash] Added a failing spec that describes issue #16
[separate-triple-dash] Corrected typos within feature
[separate-triple-dash] Added failing feature that describes issue #16
[separate-triple-dash] Started vertically separating '---'

There's now a pre-processing step when running markdown, and this
ensures that the symbol reveal-ck uses for separating slides (---) is
vertically separated from other content.

In particular, this vertical separation ensures that the --- is not
misinterpreted as a "setext" style heading.
CHANGELOG