Detect link labels without matching link reference definition

[mattias-p]

According to the latest Commonmark spec:

A shortcut reference link consists of a link label that matches a link reference definition elsewhere in the document and is not followed by [] or a link label.

Consequently syntax meant to be a shortcut reference link but whose label that doesn't match a link reference definition is not recognized as a link at all. There are a number of easy mistakes you could make to end up in this situation. You could misspell a link label, forget to add a matching definition, remove a definition that's still in use, or rename a definition but forget to update a reference to it.

It would be great if linky could emit a special tag for syntax that would be interpreted as a shortcut reference link if there was a matching link reference definition.