GeoJSON driver should pass through non-standard object members

[sgillies]

The easiest way to extend the GeoJSON format is to add new members to Feature objects. Consider styling: better not to add styling parameters to a feature's properties where they may collide with or drown out feature attributes. Currently, the OGR GeoJSON driver filters out features members not specified in the GeoJSON or EsriJSON docs (plus a couple GeoCouch-specific members) and makes new feature properties the only practical way to extend GeoJSON. This is not an ideal situation.

Let's let non-standard members pass through and help developers experiment with extending GeoJSON.