Previously we used an odd mix of 3- and 4-space indentation.
Now we use 3-space indentation, except for ordered lists,
where indentation must depend on the width of the list marker.
Closes#4563.
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.
