Support stacked charts with both detail and color

[kanitw]

Fixes #877

[kanitw]

@jpocom @domoritz ready for review.

This spec works now

{
"mark": "area",
"encoding": {
"x": {
"field": "Year",
"type": "temporal",
"timeUnit": "year"
},
"y": {
"field": "*",
"aggregate": "count",
"type": "quantitative",
"displayName": "Number of Records"
},
"color": {
"bin": true,
"field": "Acceleration",
"type": "quantitative"
},
"detail": {
"bin": true,
"field": "Displacement",
"type": "quantitative"
}
},
"data": {
"url": "data/cars.json"
}
}