fix: reference-style links are joined by gq

[Biebar]

Reference style links aren't considered in formatlistpat. This commit adds them by generalizing footnotes.

To reproduce, edit the following markdown file and use gqG :

[A][a] and [B][b] are short links.

[a]: <short link>
[b]: <short link>

You should obtain the following incorrectly formatted markdown :

[A][a] and [B][b] are short links.

[a]: <short.link/1> [b]: <short.link/2>