Support implicit and shortcut reference links

[reagle]

F12 allows me to bounce between the first link and its reference, but not the latter two:

According to <https://pandoc.org/MANUAL.html#reference-links>:

[My first link][first] is explicit, 
my [second link][] is implicit,
my [third link] is a shortcut.

[first]: /bar/baz
[second link]: /baz/bar
[third link]: /foo/bar