std logger.

DebugLevel logs are typically voluminous, and are usually disabled in production.

ErrorLevel logs are high-priority.

FatalLevel logs a message, then calls os.Exit(1).

InfoLevel is the default logging priority.

PanicLevel logs a message, then panics.

WarnLevel logs are more important than Info, but don't need individual human review.

log level string name mapping.

log level.