Implement fields extension

[jisantuc]

Improvement

For debugging and for environments where shipping a lot of extra JSON is costly (bandwidth-constrained environments, workflows where you might want enough items that the extra time / bandwidth adds up to significant overhead), it would be good to be able to limit the fields returned in search with the fields extension. The fields extension has some pretty straightforward semantics and is allowed to break the promise that search returns valid items (or even valid geojson), so it should be doable with JSON-only operations applied to the search result.

[jisantuc]

Follow up to #707