Make status information available as JSON

It would be great if we could get the status info available as JSON, perhaps with a `status-json` command. This would make things easier to parse by other tools and more future-proof as we don't have to think about how to format the data as new status fields get added.

This could also be made available to the `gh-ost-on-status` hook via a `GH_OST_STATUS_JSON` env var.

Related to #242 since adding more fields requires us to think about formatting.