Use Log4j, add console improvements and allow plugins to use SLF4 for logging

[stephan-gh]

This is a combination of moving Waterfall to Log4J (related to #139), additional console improvements using TerminalConsoleAppender (see PaperMC/Paper#728) and allowing plugins to use SLF4J (related to PaperMC/Paper#890).

It's similar to #139 but implemented differently. The only thing both PRs have in common is that Log4J2 is used (together with its async logging functionality). Otherwise they are different:

• TerminalConsoleAppender and JLine 3 are used for console improvements (e.g. colors, command completion, ...). See Improve console implementation Paper#728 for new features introduced by switching.
• Logged messages are forwarded using a custom Handler instead of Log4J's custom JUL LogManager. This is mostly because Log4J's custom LogManager prevents use of custom handlers attached to the JUL Logger and some plugins may make use of that. It's now similar to what Spigot does to forward them and I think it has shown to work quite well.
• System.out and System.err are redirected to the logger using Log4J IOStreams

There are two additional commits that are equivalent to the changes in PaperMC/Paper#890. Plugin prefixes are handled using logger names in the Log4J configuration and plugins may now use SLF4J for logging.

As far I can tell, everything is already working correctly but I still need to do some cleanup...

[stephan-gh]

As discussed on IRC, I've added a Log4j configuration file that replicates the old BungeeCord setup. It can be enabled by running Waterfall using -Dlog4j.configurationFile=log4j2-bungee.xml. However, the default is a setup that is very similar to what Vanilla/Paper/Sponge/Forge use.

[kashike]

Thanks, @Minecrell.

[jamierocks]

Stuff to be done while merging

[jamierocks]

*/ should be on the next line