Simplified universal serialization support in json.dump() can be achieved
by passing 'default=str', which was already the case in DataJob.run()
for -j/--dump-json, but not for the 'metadata' post-processor.
This commit introduces util.dump_json() that (more or less) unifies the
JSON output procedure of both --write-metadata and --dump-json.
(#251, #252)