Add formula transform.
Add high dpi (retina) support.
Clean up.
Merge pull request #23 from lgrammel/master

Unit tests for vg.array, vg.unique, vg.keys, vg.values, vg.str
Add headless server-side rendering.
Add custom axes.
Add CLI arguments.
Add configuration file.
Update zip for unequal data lengths.
Add data loading module.
Refine axis style property specification.
Add rule mark.
Add headless image loading.
Rename vars for consistency.
Add expression parser.
Add unit tests for vg.number .