public Spider thread(ExecutorService executorService, int threadNum) {
        checkIfRunning();
        this.threadNum = threadNum;
        if (threadNum <= 0) {
            throw new IllegalArgumentException("threadNum should be more than one!");
        }
        this.executorService = executorService;
        return this;
    }
start with more than one threads

@param executorService executorService to run the spider
@param threadNum threadNum
@return this
webmagic-core/src/main/java/us/codecraft/webmagic/Spider.java