[feature request]: Add PDF output

[jmetz]

Reveal has a reasonable approach to this involving a pdf-print view and printing to PDF from the browser , but it's not great!

Presumably this should be much more straight forward here along the lines of eg mdproof.

[lunchboxav]

ah, thanks for the suggestion @jmetz. mdproof looks interesting, I wonder if we can use any other ready to use library for this purpose. need to experiment.