Variables
This will be a bit of a tough one to implement right and store properly... current thoughts:
- Cascades down and can be overriden at different levels (Document, Page, Artboard, Group, Layer)
- Variables are essentially mathematic expressions assigned to something like `$a`, etc.
- Can build off previous value of a variables, so `$padding = $padding + 100%`

Possible applications:
- Change padding between layers quickly and easily
- Toggle the values of variables to test different configurations quickly
- Cuts down on common, repetitive constraints

Possible an extraneous feature. There's downsides to this being (mis)/applied as well. Feedback welcome.
