Support lab color interpolation

[tmcw]

From mapbox/mapbox-gl-style-spec#62 (comment) - there are many perceptual scales, but this is a good place to start.

[pnorman]

👍 to CIELAB or it's angular transformation, CIELch.

I'm doing all my colour picking in Lch these days

[tmcw]

Now that data-driven styles have landed, I think this is actionable.

'circle-color': {
    property: 'sqrt',
    'interpolate-color': 'lab',
    stops: [
        [0, '#f1f075'],
        [250, '#e55e5e']
    ]
}

cc @lucaswoj @jfirebaugh ?

[lucaswoj]

@tmcw Do you have thoughts on exposing a interpolate-color configuration option vs adopting a better color interpolation algorithm by default?