Custom colorscales

Plotly supports sending custom colorscales as well as the predefined ones. It might be nice to try and support that.

For instance, the following request body to the Plotly API seems to do what we expect (`POST https://api.plot.ly/v2/plots` ; you need to set the headers `Plotly-Client-Platform` to something (eg. `scala`), `Content-Type` to `application/json` and use basic auth with your username and password):

``` json
{
"figure":{
"data":[
{
"zsrc":"pbugnion:452:0049bf,2b5279,47daf1,964c04,6841d3,961eab",
"type":"heatmap",
"colorscale":[
[
7,
"rgb(165,0,38)"
],
[
7.1111111111111111,
"rgb(215,48,39)"
],
[
7.2222222222222222,
"rgb(244,109,67)"
],
[
7.3333333333333333,
"rgb(253,174,97)"
],
[
8.4444444444444444,
"rgb(254,224,144)"
],
[
8.5555555555555556,
"rgb(224,243,248)"
],
[
8.6666666666666666,
"rgb(171,217,233)"
],
[
8.7777777777777778,
"rgb(116,173,209)"
],
[
8.8888888888888888,
"rgb(69,117,180)"
],
[
9,
"rgb(49,54,149)"
]
]
}
]},
"world_readable":true
}
```

The `colorscale` field seems to be an array of 2-arrays containing a z-value and a color.

I think the following interface would work:

``` scala
// current interface in, eg. contour plots
val p = ThreeDPlot().withSurface(zs, SurfaceOptions().colorscale("Viridis"))

// extensions
val customColorscale = Coloscale.fromIterable(
List(7 -> Color.rgb(255, 0, 0), 7.5 -> Color.rgb(128, 128, 0), 8.0 -> Color.rgb(0, 255, 0))
)
val p = ThreeDPlot().withSurface(zs, SurfaceOptions().colorscale(customColorscale))

val customColorscale = Coloscale.fromFunction { z =>
if (z < 10.0) { Color.rgb(255, 0, 0) } else { Color.rgb(0, 255, 0) }
}
val p = ThreeDPlot().withSurface(zs, SurfaceOptions().colorscale(customColorscale))
```