refactor proxy handling code (#2357)

- allow gallery-dl proxy settings to overwrite environment proxies
- allow specifying different proxies for data extraction and download
  - add 'downloader.proxy' option
  - '-o extractor.proxy=–PROXY_URL -o downloader.proxy=null'
    now has the same effect as youtube-dl's '--geo-verification-proxy'

docs/configuration.rst

@@ -477,6 +477,7 @@ Description
     | Can be either a simple ``string`` with just the local IP address
     | or a ``list`` with IP and explicit port number as elements.
 
+
 extractor.*.user-agent
 ----------------------
 Type
@@ -2619,6 +2620,17 @@ Description
     Certificate validation during file downloads.
 
 
+downloader.*.proxy
+------------------
+Type
+    ``string`` or ``object``
+Default
+    `extractor.*.proxy`_
+Description
+    | Proxy server used for file downloads.
+    | Disable the use of a proxy by explicitly setting this option to ``null``.
+
+
 downloader.http.adjust-extensions
 ---------------------------------
 Type