Add contourgl trace module

[etpinard]

@mdtusz @bpostlethwaite @mikolalysenko

as for heatmapgl, contourgl won't be part of the dist plotly.js bundle - until gl2d plots can have reliable image test support.

To register contourgl :

var Plotly = require('plotly.js/lib/core');

Plotly.register(
  require('plotly.js/lib/contourgl')
);

Plotly.plot(/* as normally */)

[mdtusz]

Nice sample plot. 🍻

[mdtusz]

Sorry this ones been sitting for a while. I can't find anything wrong here though!

💃

[bpostlethwaite]

💃