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.
The `raw_attribute` will be used to mark raw bits, even HTML
and LaTeX, and even when `raw_html` and `raw_tex` are enabled,
as they are by default.
To get the old behavior, disable `raw_attribute` in the writer.
Closes#4311.
