Allow JSON encoded output to be properly forwarded to stackdriver

[withnale]

Was looking for a pipe solution for sending logs to stackdriver and found logpipe.

However, we utilise the structured logging that stackdriver provides and this expects the logs to be passed to stackdriver from JSON logs.

This PR introduces a new flag called --json which if specified:

• Tries to handle the log line as an encapsulated JSON log, unmarshalling and passing to stackdriver correctly.
• If the log line cannot be parsed as JSON will fallback to logging it normally as text