Support unmarshalling name in featureCollection

[Sanjay126]

Please add a name field in FeatureCollection struct so that we can use that data as well.

[paulmach]

Is the part of the geojson spec? I don't see it https://tools.ietf.org/html/rfc7946

I've been reluctant with adding other properties because it seems everyone uses their own. I'm not sure how to handle this cleanly in go.

[paulmach]

I have a PR to address this by adding an ExtraMembers map to the feature collection where all these extras will be unmarshalled to. See #56 for all the details.

[paulmach]

I hope this if finally solved by #56 and is in the recently tagged v0.2