MarkdownHtmlConverter spec changed.

Rouge syntax highlighting changed its containing markup format, which
broke specs that were not expecting the change. Nailing that down had
me chasing my tail for an *extended* period of time; now that I (think
I) know what to look for, I can go and re-enable the exploration spec.
Pffft.

Also added new `.rubocop.yml` file to disable new, intrusive cop. See
comments in that file.

RSpec: 705 examples, 0 failures
RuboCop: 179 files inspected, no offenses detected