Support gradients as scale ranges

Here's an example spec showing what I'd like to end up with (it fails to parse currently).

{
  "name": "fillGradient",
  "domain": {"data": "table", "field": "amount"},
  "range": {
    "gradient": "linear",
    "stops": [
      {"offset": 0, "color": "red"},
      {"offset": 0.5, "color": "white"},
      {"offset": 1, "color": "blue"}
    ]
  }
}

This should produce solid-colored bars, where each bar's color is determined by mapping its domain value onto the gradient. The goal here is mainly the stops feature of gradients: there doesn't seem to be a way to accomplish that with color scheme scales.

Here's a similar example spec using a color scheme, but without any control over the stops.