Hoedown should ignore link definitions in HTML comments

[Sjlver]

The following example renders wrongly in Hoe(xt)down:

Link should go to example.com but goes to Wikipedia: [foo]

[foo]: http://example.com/foo

<!-- commented-out link definition
[foo]: https://en.wikipedia.org/wiki/foo
-->

Most other interpreters get this right: Babelmark