Mapbox subplots open items

[etpinard]

As mentioned in #626

Limitations

• No line.dash support. I ran into some issues on restyle. I think it is an issue with mapbox-gl. Further investigation is needed.
• No marker.line styling options. mapbox-gl doesn't natively support them. I think it's worth waiting for them instead of hacking something in.
• No locations as an alternatively to lon / lat coordinates like in scattergeo.
• Non-default marker.symbol has limited support for marker.color
• Limited arrayOk support

Testing issues

• No image testing on CircleCI (running them is waay too slow - more info in #816)
• No interaction testing on CircleCI (side-effect of #241 - alleviated by #1455)