Org reader: support Pandocs citation extension

Citations are defined via the "normal citation" syntax used in markdown,
with the sole difference that newlines are not allowed between "[...]".
This is to keep consistency, as org-mode generally disallows newlines
between square brackets.

The extension is turned on by default (can be turned via the default
syntax-extension mechanism, i.e. by specifying "org-citation" as the
input format).