title: Pass extra values along with urls to scrapy spider
body: <p>I've a list of tuples in the form (id,url)
  I need to crawl a product from a list of urls, and when those products are crawled i need to store them in database under their id.</p>
  
  <p>problem is i can't understand how to pass id to parse function so that i can store crawled item under their id.</p>