[flickr] improve code and metadata

- simplify pagination
- add more metadata and slightly change its structure
  - convert suitable values to int or list
  - move keys from ["photo"] to the base level
- proper video support (#246)
- rename method and variable names to better fit with other extractors

docs/configuration.rst

@@ -529,12 +529,12 @@ Description The ``access_token`` and ``access_token_secret`` values you get
 =========== =====
 
 
-extractor.flickr.metadata
--------------------------
+extractor.flickr.videos
+-----------------------
 =========== =====
 Type        ``bool``
-Default     ``false``
-Description Load additional metadata when using the single-image extractor.
+Default     ``true``
+Description Extract and download videos.
 =========== =====
 
 

docs/gallery-dl.conf

@@ -40,7 +40,7 @@
         {
             "access-token": null,
             "access-token-secret": null,
-            "metadata": false,
+            "videos": true,
             "size-max": null
         },
         "gelbooru":