[pixiv] implement 'include' option

- split 'user' extractor and its 'avatar' and 'background' options into
  separate extractors ('artworks', 'avatar', 'background')
- avatars can now be downloaded with
  https://www.pixiv.net/en/users/ID/avatar
  as URL and will use a proper archive key; similar for backgrounds
- options for the 'user' subcategory must be moved to 'artworks' to have
  the same effect as before

docs/gallery-dl.conf

@@ -201,7 +201,7 @@
         "pixiv":
         {
             "refresh-token": null,
-            "avatar": false,
+            "include": "artworks",
             "tags": "japanese",
             "ugoira": true
         },