Add tokens to differentiate links/images w/ resources, and links/images w/ an optional reference

[wooorm]

(note: resource == [a](b), reference == [a], [a][], [a][b])

Currently, the label ([a]) and resource ((a)) or reference ([a]) are wrapped in an image or link token.
It might make sense to add a token for image- or link-with-resource and image- or link-with-reference-or-not.

Needs a good name.