Add support for Vega visualization language

[nathancarter]

Add support for Vega for drawing charts and plots.

It's "a visualization grammar, a declarative language for creating, saving, and sharing interactive visualization designs. With Vega, you can describe the visual appearance and interactive behavior of a visualization in a JSON format, and generate web-based views using Canvas or SVG."

This requires also extending ConvertDataSeriesForTool to support the vega format.

(I'm adding this issue as part of an effort to make my to-do list for this project public.)