Add aspect ratio configuration

[ianstormtaylor]

So you can choose a specific aspect ratio without having to define a specific width. For when the sketch is full screen.

[ianstormtaylor]

Alternatively… could allow for using vh/vw-style units so you could get fancier and open up more possibilities of having the screen's dimensions taken into account. Simple ones like letterboxing a 16:9 video, but also something like 50% width and 50% height?

The tricky part of this though is that dimensions are currently [number, number, Units] and in the case of viewport-specific ones... we need [50vw, 50vh] where the units are slightly different… tricky.

Maybe units should be just strings? Not sure.