Reworked the command registration system

Please read the [javadoc](https://github.com/Limeth/Terasology/blob/16965803d4edd128ce5baaa3c35e814de26752f3/engine/src/main/java/org/terasology/logic/console/Command.java).
I would suggest this should be a comma too, so from the user perspective the delimiter is consistent and varargs are hidden. This should pose no issue for processing, as a command can only have on varargs parameter at the end.