Better Markdown engine support

[preaction]

We should better support markdown engines like CommonMark.

Since they may not have the same API as Text::Markdown, we should create a wrapper class like Statocles::Markdown::CommonMark that makes the API the same.

In the configuration file, we should allow markdown: CommonMark. Create a coercion in Statocles::Types that translates a string value into an object of Statocles::Markdown::<string>.