title: Scrapy how to set DEPTH_LIMIT from a command line argument
body: <p>I currently have DEPTH_LIMIT set in the settings module for the scraper I am building.  I would like to be able to pass a depth limit in as a command line argument.  I have tried the following as a constructor for the crawler (and variations of it):</p>
  
  <pre><code>    def __init__(self, max_depth=3, *args, **kwargs):
          super(MySpider, self).__init__(*args, **kwargs)
          self.settings['DEPTH_LIMIT'] = int(max_depth)
  </code></pre>
  
  <p>but, I get an error and the stack dump ends with: </p>
  
  <pre><code>  File "/usr/local/lib/python2.7/dist-packages/scrapy/spider.py", line 41, in crawler
        assert hasattr(self, '_crawler'), "Spider not bounded to any crawler"
     AssertionError: Spider not bounded to any crawler
  </code></pre>
  
  <p>Even trying to print self.settings['DEPTH_LIMIT'] in the constructor causes and error.  How can I set the DEPTH_LIMIT in a crawler from a command line argument?</p>
  
  <p>Thanks!</p>