stopWhenComplete，增加动态修改完成时停止方法。 (#1169)

Co-authored-by: niuxiaozu <niuxiaozu@yeah.net>

webmagic-core/src/main/java/us/codecraft/webmagic/Spider.java

@@ -85,7 +85,7 @@ public class Spider implements Runnable, Task {
 
     protected AtomicInteger stat = new AtomicInteger(STAT_INIT);
 
-    protected boolean exitWhenComplete = true;
+    protected volatile boolean exitWhenComplete = true;
 
     protected final static int STAT_INIT = 0;
 
@@ -598,6 +598,13 @@ public class Spider implements Runnable, Task {
         }
     }
 
+    /**
+     * Stop when all tasks in the queue are completed and all worker threads are also completed
+     */
+    public void stopWhenComplete(){
+        this.exitWhenComplete = true;
+    }
+
     /**
      * start with more than one threads
      *