Feature: Logs into file, JSON format

[yorickdowne]

Two features actually, that would be helpful for log parsing:

Ability to log to a file without using tee, something like --logfile. Useful for exporting via AWS CloudWatch et al

Format of that log file to be JSON, for easier parsing

Suggested JSON format:

• level - info, warn, error
• ts - timestamp, example "1616876543.8821354"
• msg - the actual log message