This change introduces 'extractor.*.retries/timeout/verify' options
as a general way to set these values for all HTTP requests.
'downloader.http.retries/timeout/verify' is a way to override these
options for file downloads only and will fall back to 'extractor.*.…*
values if they haven't been explicitly set.
Also: downloader classes now take an extractor object as first argument
instead of a requests.session.
4a348990f4