headingDivider as local directive

[Chaphasilor]

Right now, headingDivider is a global directive, and can't be used locally.
This is mostly because the headingDivider setting changes how the slides are split, which in turn changes which local directives are applied to which slides.

However, it might be desirable to use the headingDivider directive for most slides, but not all of them.
For example:

If you use headings to structure your presentation, the headingDivider directive is almost perfect, but it prevents you from using multiple headings (e.g. subheadings) on your first slide.

If you look at other presentations out there, almost all use a subheading on the first slide (and maybe the last slide), but only a single heading on all other slides.

A local headingDivider directive, applied after the first slide (and maybe overwritten on the last slide) would fix this issue and make creating a standard presentation in markdown a breeze!

[yhatt]

Related: marp-team/marp#64