title: crawl a list of sites one by one with scrapy
body: <p>I am trying to crawl a list of sites with <code>scrapy</code>. I tried to put the list of website urls as the <code>start_urls</code>, but then I found I couldn't afford so much memory with it. Is there any way to set the <code>scrapy</code> crawling one or two sites at a time?</p>