Make sure we don't get duplicate reference links, even with
--reference-location=section.
% pandoc --reference-links -t markdown --reference-location=section --atx-headers # a  # b  ^D # a ![][1] [1]: a # b ![][2] [2]: b
Subsidiary issue: allow line break between reference link url/title and attributes:
% pandoc
[a]
[a]: url
{.class}
^D
<p><a href="url" class="class">a</a></p>