share extractor and downloader sessions

There was never any "good" reason for the strict separation
between extractors and downloaders. This change allows for
reduced resource usage (probably unnoticeable) and less lines
of code at the "cost" of tighter coupling.

gallery_dl/extractor/imgchili.py

@@ -27,7 +27,6 @@ class ImgchiliExtractor(Extractor):
         page = self.request(self.url, encoding="utf-8").text
         data = self.get_job_metadata(page)
         yield Message.Version, 1
-        yield Message.Headers, self.session.headers
         yield Message.Directory, data
         for url, image in self.get_images(page):
             data.update(image)