Added the quit command

This command quits the game with exit code 0.

src/main/java/org/bitbiome/commands/CommandListener.java

@@ -11,6 +11,9 @@ public class CommandListener {
         commands = new HashMap<>();
 
         commands.put("help", new HelpCommand());
+
+        commands.put("exit", new QuitCommand());
+        commands.put("quit", new QuitCommand());
     }
 
     public HashMap<String, CommandAPI> returnCommands() {

src/main/java/org/bitbiome/commands/QuitCommand.java

@@ -0,0 +1,12 @@
+package org.bitbiome.commands;
+
+import java.util.Scanner;
+
+public class QuitCommand implements CommandAPI {
+
+    @Override
+    public void performCommand(Scanner scanner, boolean isRunning, String message) {
+        System.out.println("You quitted!");
+        System.exit(0);
+    }
+}