Collection support!

Emit multiple items in a metabook collection as individual chapters
or sections.  Support the 'columns' field in the metabook specification,
and switch as necessary.  Support the title, subtitle, and summary
fields in the metabook collection object.  Support overriding the
toc settings from the CLI.

Change-Id: Ic5e12d259969c396d53af91b70fce46740950047
Merge "Collection support!"