Support for 'other members' in GeoJson all objects?

[jsiedentop]

The specification states that a geojson object may have other members. This is currently only implemented for features by propagating the fields property as Map<String, dynamic>.

3. GeoJSON Object

• A GeoJSON object represents a Geometry, Feature, or collection of Features.
  • A GeoJSON object MAY have other members (see Section 6).

Nothing we need for TurfDart, but to be compliant with the standard we should keep this in mind.