Automate SVG/PNG generation

[zuhao]

Plotrb's main focus is to generate Vega-compatible JSON spec, which can be interpreted by Vega either in browser or to produce the final SVG/PNG directly.

It would be very convenient if we can automate the latter. For this task, you can write a Ruby/shell script that feeds Plotrb's output to Vega's headless mode and generate the image directly. Users don't even need to be aware of the JSON spec.