Page保存了上一次抓取的结果，并可定义待抓取的链接内容。
主要方法：
{@link #getUrl()} 获取页面的Url
{@link #getHtml()} 获取页面的html内容
{@link #putField(String, Object)} 保存抽取的结果
{@link #getResultItems()} 获取抽取的结果，在 {@link us.codecraft.webmagic.pipeline.Pipeline} 中调用
{@link #addTargetRequests(java.util.List)} {@link #addTargetRequest(String)} 添加待抓取的链接
Store extracted result and urls to be crawled.
Main method：
{@link #getUrl()} get url of current page
{@link #getHtml()} get content of current page
{@link #putField(String, Object)} save extracted result
{@link #getResultItems()} get extract results to be used in {@link us.codecraft.webmagic.pipeline.Pipeline}
{@link #addTargetRequests(java.util.List)} {@link #addTargetRequest(String)} add urls to fetch
@author code4crafter@gmail.com
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