Add support for newline separated JSON

[thijslemmens]

In our applications we try to write all the logs to stdout in a newline separated JSON format. One applications can have different types of logs:

• access logs
• logs from the app itself
• p6spy logs

Our logging tool allows us to filter out the log we need. We can implement JSON logging for p6spy in our own library, but would a configurable JSON format be a useful feature to include in the p6spy library itself? If we make a PR, would it have a chance of being accepted?