Support exclusive custom logging levels using negative numbers.

Would like to set up exclusive custom levels so messages are only written to matching transports.  Any other transport with a level < log level are ignored.

     //set my custom log levels    
     logger.setLevels({error_only:-3,warn_only:-2,info_only:-1});
    
     //log only to custom warnings transport, not any other levels <
     logger.log('warn_only','Only write this message to transport configured with     level:warn_only');

Basically if a custom level is set with a negative number, the transport level needs to equal the logging level, instead of <= logging level.  Can be mixed with levels >= 0.