LuaLogging: A simple API to use logging features in Lua

LuaLogging
A simple API to use logging features in Lua.

## Nginx/Openresty appender

This appender has no configuration options, and simply forwards whatever is being logged to the configured Nginx log. LuaLogging log-levels will be converted to the equivalent Nginx ones, and initial log-level will be set to match the Nginx system level.

It makes sense to configure this logger and set it as the default logger on Nginx/OpenResty startup. Any libraries will then be able to just grab the defaultLogger from `logging.defaultLogger()` and won't need any modifications to work with Nginx/OpenResty.

## Examples

    require("logging.nginx")
    local logging = require("logging")

    local logger = logging.nginx()
    logging.defaultLogger(logger)

    logger:info("logging.nginx test")
    logger:debug("debugging...")
    logger:error("error!")