lgl is a simple logger supporting log level.

• Work out of the box.
• Only global functions are provided. Just import the package, then you can use it.
• Supported log levels: Trace, Debug, Warning, Error.
• By default show the file name and line number.
• By default log level is Error. Call SetLogLevel() in main() to enable more log.
• Call Set() in main() to save log to a file or other location.