title: dynamic start_urls in scrapy
body: <p>I'm using scrapy to crawl multiple pages on a site.
  The variable <code>start_urls</code> is used to define pages to be crawled. 
  I would initially start with 1st page, thus defining <code>start_urls = [1st page]</code> in the file <code>example_spider.py</code></p>
  
  <p>Upon getting more info from 1st page, I would determine what are next pages to be crawled, then would assign <code>start_urls</code> accordingly. Hence, I have to overwrite above example_spider.py with changes to <code>start_urls = [1st page, 2nd page, ..., Kth page]</code>, then run scrapy crawl again.</p>
  
  <p>Is that the best approach or is there a better way to dynamically assign <code>start_urls</code> using scrapy API without having to overwrite <code>example_splider.py</code>?
  Thanks.</p>