structured logging

[ctcjab]

Adding support for json structured logging would enable supercronic users to easily have logs consumed by Datadog (and similar platforms) such that the data in each log message is properly preserved as separate fields (log level, time, message, any extra params).

I read the https://github.com/aptible/supercronic#logging docs but didn't see anything about structured logging (or otherwise customizing the log format). See also #183.

[ctcjab]

Note, not having things like the log level picked up properly means that downstream things like monitoring and alerting are harder to set up as well (e.g. when a threshold of error logs is reached).