add headers as an option

[eparejatobes]

for courses, meetings etc we need to put a banner/header on top of the slides, displaying our name/logo and maybe something more (title, whatever).

I'd like to keep this as an option, and as the real motivation is that you want a slideshow for either era7 bioinformatics or oh no sequences!, it looks reasonable to just add two switches: --ohnoseq and --era7, for example.

I need to do the design, and implement that in the template.