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.
This avoids overly narrow labels for ordered lists with
() delimiters.
However, arguably it creates overly wide labels for bullets.
Also, lists now start flush with the margin, rather than
indented.
Fixes#2421.