Deprecate --base-heading-level.
The new option does everything the old one does, but also
allows negative shifts. It also promotes the document
metadata (if not null) to a level-1 heading with a +1 shift,
and demotes an initial level-1 heading to document metadata
with a -1 shift. This supports converting documents that
use an initial level-1 heading for the document title.
Closes#5615.
