Possible Presento theme improvements

[eddelbuettel]

In no particular order, and in comparison to the upstream repo:

• Full color pages :-( (as also noted in Possible Metropolis theme improvements #1); this may need some pandoc and or beamer changes
• Examples are pretty heavy on LaTeX; Markdown has e.g. no centering feature
• Maybe offer default fonts? OTOH Presento is pretty tied to its set