Reference-style links to .md files aren't resolved to the generated url

[Firehed]

Using a reference-style link fails to automatically convert a markdown file url to its html counterpart, which is inconsistent with the documentation indicating that links should work between pages.

That is to say:

Using [a link](foo.md) to [a page][bar] or [baz][]

[bar]: bar.md
[baz]: baz.md

renders

Using <a href="foo.html">a link</a> to
<a href="bar.md">a page</a> or <a
href="baz.md">baz</a>

instead of

Using <a href="foo.html">a link</a> to
<a href="bar.html">a page</a> or <a
href="baz.html">baz</a>

In practice it's not really a huge issue, but it's not quite what you would expect to happen.

[mnapoli]

Right, I'm guessing that this class only rewrites classic links and not reference-style links. I'm guessing #134 (i.e. replace Parsedown with CommonMark) would help since we could get rid of the regex and directly edit the AST that was generated from the markdown source. But for that we need to have a CommonMark extension for Markdown Extra, because currently that's a feature offered by Couscous.

[mnapoli]

On the "CommonkMark" topic I've opened #156.