External module loader

[zzzowoey]

Being able to load modules externally (i.e. from a folder next to the .jar) would be a great extension of the module system. Theoretically, implementing this shouldn't be terribly hard with the way that module loading and invocation works right now; at the very least, we'd have to make moduleClasses in CommandRegistry a class-level member and add a tryLoadModule() method. Dynamically loading and unloading module files at runtime would easily be possible with the NIO WatchService API.