Add support for Mapbox SimpleStyle

[stefanb]

Spec: https://github.com/mapbox/simplestyle-spec

The most popular feature marker-color is the easiest to implement.

[vohtaski]

I would be also very interested by this.
In my particular case, I need to color different types of lines for analysis.

{
    "type": "FeatureCollection",
    "features": [{ 
        "type": "Feature",
        "geometry": {
            "type": "LineString",
            "coordinates": [[10, 10], [20, 20]]
        },
        "properties": {
            "stroke": "#ff0000",
            "type": "short"
        }
    }, {
        "type": "Feature",
        "geometry": {
            "type": "LineString",
            "coordinates": [[0, 0], [100, 100]]
        },
        "properties": {
            "stroke": "#00ff00",
            "type": "long"
        }
    }]
}