Allow clients to specify label styling

[nickgaray]

Label styling, such as color and optionally border should be allowed for any PointMarkerLayer.

[nickgaray]

The example image provided shows a black label with white outline

[mdhsl]

I agree, the biggest difficulty is to have something homogeneous between the different views. But for now, we can add the properties in the Layer and implement them in the Cesium view

[alexrobin]

Instead of doing something specific for Cesium label outlines, I think it would be better to have a general mechanism to add any custom configuration that is specific to a particular view implementation (i.e. not handled by generic styler properties). It could be done via a callback providing the graphical object created by the view impl for example, so it can be further customized.