Allow GitLab references to work even when `autolink: false`

This depends on the Label MR work, so I will have to merge that first.

The solution here is not ready to be shipped as it slows down to as slow as when using autolink: true.

There is a workaround which I'm trying to implement:

1. Remove the namespace from the regexp
2. Look for the start symbol @
3. Run the expensive regexp and attempt to backtrack to retrieve the namespace

Closes #115 (closed)