This is now the default for pandoc's Markdown.
It allows whitespace between the two parts of a
reference link: e.g.
[a] [b]
[b]: url
This is now forbidden by default.
Closes#2602.
Modified template to include a `<back>` and `<body>` section.
This should give authors more flexibility, e.g. to put
acknowledgements metadata in `<back>`. References are
automatically extracted and put into `<back>`.
