[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/supportedsites.md

@@ -604,7 +604,7 @@ Consider all sites to be NSFW unless otherwise known.
 <tr>
     <td>Pixiv</td>
     <td>https://www.pixiv.net/</td>
-    <td>Favorites, Follows, pixiv.me Links, pixivision, Rankings, Search Results, Sketch, User Profiles, individual Images</td>
+    <td>Artworks, Avatars, Backgrounds, Favorites, Follows, pixiv.me Links, pixivision, Rankings, Search Results, Sketch, User Profiles, individual Images</td>
     <td><a href="https://github.com/mikf/gallery-dl#oauth">OAuth</a></td>
 </tr>
 <tr>