Make the log formatter configurable

[mtrop-godaddy]

Apologies for not opening a feature request first. Just wanted to see how easy it was to implement support for JSON logs and it ended up being fairly simple.

Any chance this could be merged? I'm happy to make adjustments if you see any issues with the current implementation. The main goal is to be able to configure sshpiper to output logs in JSON format (by using logrus's JSONFormatter instead of the default Text one).