We used to insert extra spaces to ensure that the content respected
the four-space rule. That is not really necessary now, since pandoc's
markdown and most markdowns don't follow the four-space rule.
Those who want the old behavior can obtain it by using
`-t markdown+four_space_rule`.
Closes#7172.
When a block only has a single class and no other attributes,
it is not necessary to wrap the class attribute in curly braces –
the class name can be placed after the opening mark as is.
This will result in bit cleaner output when pandoc is used
as a markdown pretty-printer.
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.
John MacFarlane