Added support for an update on load feature for plugins. Thanks Raphfrk!

Any files placed in the new (optional) update folder are automatically copied into the plugins directory the next time a reload happens. This allows safe updating of the plugin .jar files.

By: EvilSeph <evilseph@unaligned.org>

paper-api/src/main/java/org/bukkit/Server.java

@@ -83,6 +83,14 @@ public interface Server {
      */
     public int broadcastMessage(String message);
 
+    /**
+     * Gets the name of the update folder. The update folder is used to safely update
+     * plugins at the right moment on a plugin load.
+     *
+     * @return The name of the update folder
+     */
+    public String getUpdateFolder();
+
     /**
      * Gets a player object by the given username
      *