consider var(--foo) a color

[mbostock]

Fixes #765. It could be stricter (checking that the variable name starts with -- for example), but I think this is fine as is.

Plot.plot({
  style: "--red: red;",
  y: {
    grid: true
  },
  marks: [
    Plot.areaY(AAPL, {x: "Date", y: "Close", fillOpacity: 0.1}),
    Plot.line(AAPL, {x: "Date", y: "Close", stroke: "var(--red)"}),
    Plot.ruleY([0])
  ]
})