Expose snapshots in REST API

[westonruter]

The snapshot posts should be available via the REST API, including the UUID for each snapshot and its contents. Accessing snapshots would require customize capability at least, and potentially the capability for each setting contained in the snapshot.

This should include a collection param for filtering snapshots that are authored by a given user ID.

Relates to #12

[westonruter]

We should get post content storage nailed down first in #25.

Then, the REST API endpoints for snapshots should override the normal content field with one that contains all of the decoded JSON data, instead of presenting encoded JSON data in a content string.

[westonruter]

Closing in favor of https://github.com/WP-API/wp-api-customize-endpoints