title: Load Item fileds with ItemLoader across multiple responses
body: <p>This is a followup question to accepted answer to question <a href="https://stackoverflow.com/questions/39681269/scrapy-populate-items-with-item-loaders-over-multiple-pages">Scrapy: populate items with item loaders over multiple pages</a>. I want to use <code>ItemLoader</code> to collect values from multiple requests to a single <code>Item</code>. The accepted answer suggests that the loaded <code>Item.load_item()</code> should be passed to the next request via <code>meta</code> field in <code>request</code>.</p>
  
  <p>However, I would like to apply <code>output_processors</code> to all collected values of a single field when returning the loaded object at the end of the crawl.</p>
  
  <h3>Questions</h3>
  
  <ol>
  <li>What would be the best way to achieve it?</li>
  <li>Can I pass the <code>ItemLoader</code> instance over <code>meta</code> to next <code>request</code> without loading it and then just replace the <code>selector</code> or <code>response</code> elements in the <code>ItemLoader</code> when adding the values or xpaths from the next response?</li>
  </ol>
  
  <p>Example:
  </p>
  
  <pre class="lang-py prettyprint-override"><code>def parse(self, response):
      loader = TheLoader(item=TestItems(), response=response)
      loader.add_xpath('title1', '//*[@id="firstHeading"]/text()')
      request = Request(
          "https://en.wikipedia.org/wiki/2016_Rugby_Championship",
          callback=self.parsePage1,
          meta={'loader': loader},
          dont_filter=True
      )
      yield request
  
  def parsePage1(self, response):
      loader = response.meta['loader']
      loader.response = response
      loader.add_xpath('title1', '//*[@id="firstHeading"]/text()')
      return loader.load_item()
  </code></pre>
  
  <p>Ignore the context of the actual websites.</p>