Previously we relied on markdown before 3.0 due to incompatible changes.

A lot of changes were required due to our previous hacky use of the
`ExtendedLinkPattern` (misusing its `sanitize_url` method for a two-step
URL resolution).

The only questionable change is probably our random selection of the
priority (100) when registering a markdown extension.
Maybe this could prove to break some complicated conversions.