Support JSON output for `fauna schema status`

[jrodewig]

We should support JSON or a similar machine-readable output for the fauna schema status command. This would make it easier to use the output of the command in scripts.

Relates to #580

[ecooper]

Do we know what we'd expect the JSON structure to be? Is --json just a pass-through of the schema APIs or something more refined?

[jrodewig]

A pass-through of the Get staged schema status endpoint's response would suffice IMO.

In the requests I've seen, users want to programmatically access the status field without doing text manipulation, etc.