Prepare Markdown feature spec for testing multiple pipelines

This MR lays the groundwork for including or excluding certain HTML::Pipeline filters depending on what we're parsing. For example, excluding reference filters in the Issues and Merge Requests titles.

The majority of this was breaking up the various filter-related specs into custom matchers so that we could, for example, do things like this:

it 'includes Emoji, but not Autolinks' do
  expect(the_html).to parse_emoji
  expect(the_html).not_to create_autolinks
end