New graphic type: ai2html

[blakebutcher]

http://ai2html.org/

It has been implemented in the nprapps repo since we forked, but we'll need a way to allow adding the static assets (html + images) so potentially via Google Sheets...

https://github.com/nprapps/dailygraphics#creating-an-ai2html-graphic
https://github.com/nprapps/dailygraphics/tree/master/graphic_templates/ai2html_graphic