Add fabric.js demo

[Cellivar]

The fabric.js project is a fairly straightforward canvas-based editor. It shouldn't be very difficult to create a demo page with a basic editor with immediate print-to-label capabilities.

I know I personally would use this, but I am also single-handedly pulling up the average number of label printers in a US household.

At the very least it'd be an interesting learning project and another testbed for integrating my library with something else.