Add support for the output suite

[Julian]

Once json-schema-org/JSON-Schema-Test-Suite#619 lands, we should add support for it, either to bowtie suite itself, or perhaps to a new bowtie output-suite command.

Implementation harnesses will need updating, both to indicate what output formats they support as well as to return output in the specified format, and we can perhaps make use of the basic output format by default and do away with the valid: "custom" way of indicating output.