Named and customized logger instance, unit tests, performance improvement and fixes

[jxai]

• Supports creating an independent instances with its own config, and optionally a name (if specified the name appears in log output):

local logger = log{name="mymod", level="warn", usecolor=false}
logger.info("my info log")

Original global logger still works exactly as it used to be

• Added a suite of unit tests, just run lua test.lua
• Disabled log levels are automatically assigned a noop function instead of checking the level on every call - more efficient and it is transparent to the user
• Other minor fixes