Add getShutdownMessage() and stop command arguments. Adds BUKKIT-3031

By: feildmaster <admin@feildmaster.com>

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

@@ -666,6 +666,13 @@ public interface Server extends PluginMessageRecipient {
      */
     String getMotd();
 
+    /**
+     * Gets the default message that is displayed when the server is stopped
+     *
+     * @return the shutdown message
+     */
+    String getShutdownMessage();
+
     /**
      * Gets the current warning state for the server
      *