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.