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 for consistency, as org-mode generally disallows newlines
between square brackets.

The extension is turned on by default and can be turned off via the
default syntax-extension mechanism, i.e. by specifying "org-citation" as
the input format.