Add a way to specify a json schema

[avh4]

It would be great if this API provided a way to encourage validation of all generated fixtures against a json schema. Even better (see https://noredink.slack.com/archives/C0VVDLEES/p1522936582000477 (private NRI)), it would be great to have a verify mode of the CLI to run on CI to verify that all produced fixtures are currently up to date with the specified json schema.

Maybe this could be an added argument to produce_elm_file ?