Property based test based on markdown-it (commonmark).

[zombiecalypse]

Note that the test is currently failing, so merge with caution

This change checks that

markdown_it.render(turndown.render(html)) ~ html

and it currently finds various corner-cases:

• "<em>foo<br/></em>bar" => "_foo \n_bar" => "_foo<br/>_bar"
• "<h1>foo<br/></h1>bar" => "# foo \nbar" => "<h1>foo</h1>bar"
• "<em>foo</em>bar" => "_foo_bar" => "_foo_bar"
• "<em><em>foo</em></em>" => "__foo__" => "<strong>foo</strong>"
• ...

The test should be easy to extend to other scenarios that may prove to be problematic.