I would love a "Side Note" block

It can be nice to have an easy way to add asides or related diagrams to articles. For example here's a screen shot of side notes I've used in the past when [writing about technical topics outside of the Cloud Four blog](http://paulhebertdesigns.com/web_colors/):

(I saw [Tufte CSS](https://edwardtufte.github.io/tufte-css/) over the weekend which also got me thinking about this.)

I wonder if we could make a block to support this use case? My assumption would be that on small screens it would switch to displaying inline in the text?

This might be simpler to build than footnote support while allowing a similar functionality.