Adds subcategories to the appenders and loggers. Adds property "level" at the file appender to limit the levels that a file appender accepts.

Creates a MARK category that always write to the log. That's useful to write things like '---- STARTED ----'.

lib/levels.js

@@ -63,6 +63,7 @@ module.exports = {
   WARN: new Level(30000, "WARN"), 
   ERROR: new Level(40000, "ERROR"), 
   FATAL: new Level(50000, "FATAL"), 
+  MARK: new Level(Number.MAX_VALUE-1, "MARK"), 
   OFF: new Level(Number.MAX_VALUE, "OFF"), 
   toLevel: toLevel
 };