Add heatmapgl trace module

[etpinard]

@mdtusz @bpostlethwaite @mikolalysenko

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

To register heatmapgl :

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

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

Plotly.plot(/* as normally */)

[mdtusz]

https://www.youtube.com/watch?v=G_f8ayQQx4I

[mdtusz]

💃

[hoidn]

Thank you for this very promising feature.

I was curious whether it would be straightforward for one to modify plolty.js and plolty.py to make it available from the python API? I rebuilt the plotly.js bundle after adding

require('./heatmapgl')

to the argument list for Core.register in lib/index.js. Now I suppose it's necessary to add heatmapgl to the json schema on the python end, but I don't know how to go about that.

[etpinard]

@hoidn here's how I would go about running a custom plotly.js bundle in plotly.py: https://gist.github.com/etpinard/21eb47feaef0acfde93cea37c6a3710a