Allow Polygons to be labelled

[chris-little]

See previous CovJSON issue covjson/specification#94

[ThorodanBrom]

Would it be reasonable to extend this to Points as well?

[jonblower]

Yes, I think so

[chris-little]

The gist of the previous closed and archived issue is that rather than attach a colour property/label to a polygon, it is preferred to attched a full Feature JSON object. There was discussion about the problems of GeoJSON objects and the implied CRS. This perhaps suggests that a Feature that is a JSON-FG would be neater and much more flexible.