12-factor app logger (library)

 Log XI is a structured 12-factor app (http://12factor.net/logs)
 logger built for speed and happy development.
 .
  * Simpler. Sane no-configuration defaults out of the box.
  * Faster. See benchmarks vs logrus and log15.
  * Structured. Key-value pairs are enforced. Logs JSON in production.
  * Configurable. Enable/disable Loggers and levels via env vars.
  * Friendlier. Happy, colorful and developer friendly logger in
    terminal.
  * Helpful. Traces, warnings and errors are emphasized with file, line
    number and call stack.
  * Efficient. Has level guards to avoid cost of building complex
    arguments.