Drawing traces on top of shapes

[zerweck]

I would like to use shapes in the background to highlight areas, without resorting to opacity.
The developer of the R API states that this is not supported in plotly.js:
plotly/plotly.R#370

[etpinard]

@zerweck This would be useful addition.

Unless @alexcjohnson has a reason for always keeping shapes above traces.

Possible API:

• layout.shape.layer: 'above' || 'below'