#535 修复Tomcat 不能正常关闭的问题，增加线程池shutdown相关的操作

weixin-java-mp/src/main/java/me/chanjar/weixin/mp/api/WxMpMessageRouter.java

@@ -75,6 +75,29 @@ public class WxMpMessageRouter {
     this.exceptionHandler = new LogExceptionHandler();
   }
 
+  /**
+   * <pre>
+   * 使用自定义的 {@link ExecutorService}
+   * </pre>
+   */
+  public WxMpMessageRouter(WxMpService wxMpService, ExecutorService executorService) {
+    this.wxMpService = wxMpService;
+    this.executorService = executorService;
+    this.messageDuplicateChecker = new WxMessageInMemoryDuplicateChecker();
+    this.sessionManager = new StandardSessionManager();
+    this.exceptionHandler = new LogExceptionHandler();
+  }
+
+  /**
+   * <pre>
+   * 如果使用默认的 {@link ExecutorService}，则系统退出前，应该调用该方法。
+   * </pre>
+   */
+  public void shutDownExecutorService() {
+    this.executorService.shutdown();
+  }
+
+
   /**
    * <pre>
    * 设置自定义的 {@link ExecutorService}