[FR] Add support for JSON formatted logs

[yorickdowne]

Feature Request: Add a --log-format or similar global that can take the value JSON. If present, logs are output in JSON format; if omitted, they are output in plain format as now.

For this to be maximally useful, logs should have a level as well. This is not currently present and described in #174 .

Rationale: Parsing for node/watchtower failure is 10x easier when the logs are JSON-formatted. Log parsers can handle cleartext but are usually optimized UX-wise for JSON logs.