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

By: feildmaster <admin@feildmaster.com>

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

@@ -383,6 +383,10 @@ public final class Bukkit {
         return server.getMotd();
     }
 
+    public static String getShutdownMessage() {
+        return server.getShutdownMessage();
+    }
+
     public static WarningState getWarningState() {
         return server.getWarningState();
     }