Author a schema for GeoJSON format

[duncandewhurst]

We should consider authoring a schema for the GeoJSON format. It could be generated programmatically from the schema for the JSON format. Doing so would improve the experience for users who submit GeoJSON files to CoVE and would reduce the amount of extra code required to handle errors in the GeoJSON format.

[duncandewhurst]

https://github.com/geojson/schema may be relevant