consolidate functions

[ansis]

We have three different ways of writing functions. Some of them are pretty complex. I think we can consolidate them into one type:

"line-width": {
    "stops": [[2, 2], [10, 5], [15, 10], [18, 12]],
    "base": 1.75 // optional
}

This is exactly like the current stops function, except the interpolation is exponential. It interpolates based on an curve that goes through the two points and has the given exponential base. Using a small base like 1.01 makes the interpolation linear.

We should do this for v4.

The math:

var a = stops[i];
var b = stops[i + 1];
var zoomDiff = b[0] - a[0];
var valueDiff = b[1] - a[1];
var slope = valueDiff / Math.pow(base, zoomDiff);
return Math.pow(base, currentZoom - a[0]) * slope + a[1];

@nickidlugash @edenh @kkaefer @mourner @yhahn @lbud

[jfirebaugh]

@lbud Want to grab this one next?

[mourner]

Should we keep the min function? Is it useful?

[lbud]

I'm going to leave this to @ansis to export it from his brain > javascript to see it in action, and then I'll grab migration things once it works

[jfirebaugh]

Done in #99.