New (moved) MarkdownHtmlConverter service class.

It's not just for `PostDataDecorator`.

Pinging Issue #74.

RSpec: 507 examples, 0 failures
RuboCop: 131 files inspected, no offenses detected
Using new MarkdownHtmlConverter service object..

Closes Issue #74.

This replaces earlier
`PostDataDecorator::SupportClasses::ContentConverter`. API and logic
virtually identical.

RSpec: 516 examples, 0 failures
RuboCop: 133 files inspected, no offenses detected
Merge pull request #75 from jdickey/issue-74

Fixes Issue #74.