Allow configuration of initial section numbering via notebook metadata

[phelps-sg]

Is your feature request related to a problem? Please describe.
I would like to give smaller presentations on subsections of a larger book (itself comprised of multiple notebooks for each chapter), while providing consistent section numbering between the slides and the books. Currently the only way to do this is to merge all notebooks before converting to slides, but the resulting presentation consists of hundreds of slides which causes poor performance with reveal.js.

Describe the solution you'd like
I would like to be able to configure the initial section numbering when converting a notebook to slides using nbpublish -f slides_ipypublish_all so that I can give a presentation on an individual chapter, with section headings corresponding to the same section headings in the book. Ideally the start section number should be configured via the same notebook metadata as used in the toc2 notebook extension; i.e. toc.base_numbering.

Describe alternatives you've considered
Merge all notebooks before converting to slides, but the resulting presentation consists of hundreds of slides which causes poor performance with reveal.js.