Add support to parse custom JSON logs

[niamster]

What would you like to be added:

I'd like to add a support for parsing logs in JSON format and re-format for better human perception.
Particularly I'm interested in logs generated by https://github.com/uber-go/zap

Why is this needed:
Everyone is leaning to generate logs in JSON, this is great, it's easier to parse them for machines.
This is less human-friendly though.
And stern is for humans, even if application logs are machine-friendly.

Proposed PR #228