Optional JSON output format from `log` module

[mbland]

Per @JohnOmernik, this will enable more robust log processing by powerful tools.

I'm pretty this option should be at least optional per log level. I'm also considering making it optional per output file, so that a regular log message can be written to the console or a file, and a JSON representation of the same message can get sent to another file.

Might be worth adding jq to the process:

• http://blog.librato.com/posts/jq-json
• https://stackoverflow.com/questions/38860529/create-json-using-jq-from-pipe-separated-keys-and-values-in-bash

[mbland]

v1.3.0