Previously we used Setext (underlined) headings by default.
The default is now ATX (`##` style).
* Add the `--markdown-headings=atx|setext` option.
* Deprecate `--atx-headers`.
* Add constructor 'ATXHeadingInLHS` constructor to `LogMessage` [API change].
* Support `markdown-headings` in defaults files.
* Document new options in MANUAL.
Closes#6662.
Instead rely on the markdown writer with appropriate extensions.
Export writeCommonMark variant from Markdown writer.
This changes a few small things in rendering markdown,
e.g. w/r/t requiring backslashes before spaces inside
super/subscripts.
Previously we got a crash, because we were trying to print
a native cmark STRIKETHROUGH node, and the commonmark writer
in cmark-github doesn't support this. Work around this by
using a raw node to add the strikethrough delimiters.
Closes#4038.