[danbooru] add 'external' option (closes #1747)

docs/configuration.rst

@@ -806,17 +806,15 @@ Description
     Download embedded videos hosted on https://www.blogger.com/
 
 
-extractor.danbooru.ugoira
--------------------------
+extractor.danbooru.external
+---------------------------
 Type
     ``bool``
 Default
     ``false``
 Description
-    Controls the download target for Ugoira posts.
-
-    * ``true``: Original ZIP archives
-    * ``false``: Converted video files
+    For unavailable or restricted posts,
+    follow the ``source`` and download from there if possible.
 
 
 extractor.danbooru.metadata
@@ -831,6 +829,19 @@ Description
     Note: This requires 1 additional HTTP request for each post.
 
 
+extractor.danbooru.ugoira
+-------------------------
+Type
+    ``bool``
+Default
+    ``false``
+Description
+    Controls the download target for Ugoira posts.
+
+    * ``true``: Original ZIP archives
+    * ``false``: Converted video files
+
+
 extractor.derpibooru.api-key
 ----------------------------
 Type

docs/gallery-dl.conf

@@ -52,8 +52,9 @@
         {
             "username": null,
             "password": null,
-            "ugoira": false,
-            "metadata": false
+            "external": false,
+            "metadata": false,
+            "ugoira": false
         },
         "derpibooru":
         {