Port Typed Feature Collection

The commit on the GeoJSON.Net libary has the [Typed FeatureCollection](https://github.com/GeoJSON-Net/GeoJSON.Net/commit/a7a828d27b19b487e9e89d0f81d5ca0cc8c8a234) but it has not been ported here. 

Please can this be done as its quite useful i ended up making a temp extension for my work until this is solved.

Thanks