Captions used to have the word 'caption' prepended; this
has been removed.
Also, 'thumb' is used instead of 'frame' to allow images
to be resized.
Closes#5105.
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.
