Configure Markdown engine using middleware pattern

This gives us benefit of ease to chain Markdown-It (or other engines) with sanitize-html.

And also, allow devs to easily reuse our configuration of sanitize-html.

() => next => ({ contentType, text }) => {
  return next({
    contentType: 'text/html',
    text: MarkdownIt.render(markdown)
  });
}

We also need to make sure if the Markdown is not processed by the middleware, we should spit out text/plain instead. So no one can easily enable XSS accidentally.

No files, entry points, or tests are named. Start by locating the Markdown rendering and sanitize-html integration, then trace how rendered content is passed through the response pipeline. Done means Markdown engines can be chained as middleware, sanitize-html configuration can be reused, and unprocessed Markdown is returned as text/plain.