in area and line marks, reduce grouped styles and titles

[Fil]

closes #503

a typical example would be:

Plot.line(data, {stroke: {value: "value", reduce: "median"}, title: {value: "value", reduce: "mode"}, x: …})

Note: a solution to #503 with the current Plot is:

Plot.line(data, Plot.map({stroke: d => {
  const r = d3.median(d);
  return Array.from(d, () => r);
}}, {stroke: "value", x: …})

shorter version (changing only the first element of the channel, since we know that's what's used later):

Plot.line(data, Plot.map({stroke: d => (d[0] = d3.median(d), d) }, {stroke: "value", x: …})

This PR introduces a slightly simpler method to call Plot.map. It may not be worth the extra code, documentation, and might be replaced by a link to a good notebook demonstrating Plot.map for this use case!

[Fil]

See #724.