Add support for go generics in geojson Feature

[robert-malhotra]

Looking at the code for geojson features it expects a map[string]interface{} for defining properties. I would like to (de)code directly to a properties struct that I define. I think those could easily be done with go generics while maintaining backwards compatibility.

I would be happy to take a stab at implementing this feature if it fits with the goals of the project.

[twpayne]

I would be happy to take a stab at implementing this feature if it fits with the goals of the project.

Please do!