Re-evaluate LOG4J output

[pvdbosch]

The LOG4J output option actually uses slf4j (not log4j). For maven users, the "simple" slf4j implementation is used by default which logs to console. The only difference with CONSOLE option is that the format is a bit different, but they're both going to console.

Maybe drop the option from the maven plugin? Or rename it to "SLF4J" or "LOGGER"?
Implementation could still be kept; it may be useful when using the "core" and "rules" modules as libraries, rather than as Maven plugin.