Add AsciiDoc attribute or theme key to enable fullscreen page mode

[mojavelinux]

A PDF document can be configured to launch in fullscreen mode. It should be possible to activate this feature using either an AsciiDoc attribute or theme key, perhaps both.

Since this setting is a switch, it makes sense to use a theme key like:

page:
  fullscreen: true

We could also consider using the lower-level terminology of mode:

page:
  mode: fullscreen

The corresponding AsciiDoc attribute would either be pdf-fullscreen or pdf-mode=fullscreen.

This sets the following property in the PDF catalog

/PageMode /FullScreen

When fullscreen is active, the existing PageMode property must be moved to:

/NonFullScreenPageMode /UseOutlines

[mojavelinux]

I'm leaning more towards page_mode: fullscreen and pdf-page-mode=fullscreen.