Add top-level key self defining this format

[emthompson-usgs]

@ellenyu @cbworden @mhearne-usgs
It occurred to us that we should have added a key that lets the parser know what format this is, more specifically than geojson.

First idea would be:

{
    "type": "FeatureCollection",
    "format": "gmp",
    "version": "1.0",
    ...
}

Thoughts? Alternatives?