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.