Explore ways to handle custom properties

[JackWink]

Since go is statically typed, custom properties are not really supported, neither are they part of the GeoJSON spec.

However, geobuf/pygeobuf are dynamically typed and therefor encode extra properties on any of the json objects.

Finding a way to expose or pull out the custom properties would be nice, but non-essential for the initial release (as they aren't part of the spec). However, if I can find a nice way to support it with minimal effort, I'll go for it.

The best way I can think of right now is to support it in decode only, perhaps returning it with type loss or with a provided schema.