Added command registration to plugin config file. Registered commands are sent to the corresponding plugin.onCommand method when executed.

By: VictorD <victor.danell@gmail.com>

paper-api/src/main/java/org/bukkit/plugin/Plugin.java

@@ -3,6 +3,7 @@ package org.bukkit.plugin;
 
 import java.io.File;
 import org.bukkit.Server;
+import org.bukkit.entity.Player;
 import org.bukkit.util.config.Configuration;
 
 /**
@@ -61,4 +62,9 @@ public interface Plugin {
      * Called when this plugin is enabled
      */
     public void onEnable();
+    
+    /**
+     * Called when a command registered by this plugin is received.
+     */
+    public void onCommand(Player player, String command, String[] args);
 }