Support for MultipolygonField

[heronrs]

Hey guys, just came accross the need for a PolygonField representation.

I tried hacking my way into it by looking at #58 and it seemed very straightforward.

Could give it a shot and send PR if that's OK

[abawchen]

@heron182

Thanks for the feedback! 👍

I just refactored the way to resolve PointField into type and coordinates automatically, that we don't have to call json.load to parse PointField.

You can refer #64 for the detail -- it would be more complex than converting it into json string, but I think it would be more friendly for the usage.

[heronrs]

Great @abawchen , I'll send a PR as soon as I can :)