Allow shortcut reference link before unmatched label

[mgeier]

This is of course an edge case. I still think the spec should be changed to be more consistent.

This concerns example 540 from the spec 0.27:

[foo][bar][baz]

[baz]: /url1
[foo]: /url2

Everywhere else, reference links are only recognized if they are matching. Unmatched reference links don't have any significance to the parser.

Therefore, in this case, the unmatched reference link [foo][bar] should also not have any significance, and the parser should continue to happily match [foo] as shortcut reference link.