Overhaul config.yml command definition to look like CommandAPI's CommandTree

[willkroboth]

The CommandAPI offers the CommandTree class as an alternative to the CommandAPICommand when registering commands. The syntax for using it to make a command fits the syntax of a yml file, so I think a new command definition system can be built off of the CommandTree.

For example, the following yml file:

commands:
  sayHi:
    permission: greet
    then:
      permission: greet.other
      name: <target>
      type: Player
      executes:
        - do <target>.sendMessage(<sender>.toString().join(" says Hi!"))
    executes:
      - do <sender>.sendMessage("Hi!")

would be roughly equivalent to this CommandTree:

new CommandTree("sayHi")
        .withPermission("greet")
        .then(new PlayerArgument("target")
                .withPermission("greet.other")
                .executes((sender, args) -> {
                    ((Player) args[0]).sendMessage(sender.getName() + " says Hi!");
                })
        )
        .executes((sender, args) -> {
            sender.sendMessage("Hi");
        })
        .register();

This new command-definition system would make it easier for users to create subcommands, and make it easier to develop new features from the CommandAPI.