[gelbooru] add support for api_key and user_id (#2767)

docs/configuration.rst

@@ -1313,6 +1313,18 @@ Description
     * ``"new"``: Expect the *new* site layout
 
 
+extractor.gelbooru.api-key & .user-id
+-------------------------------------
+Type
+    ``string``
+Default
+    ``null``
+Description
+    Values from the API Access Credentials section found at the bottom of your
+    `Account Options <https://gelbooru.com/index.php?page=account&s=options>`__
+    page.
+
+
 extractor.generic.enabled
 -------------------------
 Type

docs/gallery-dl.conf

@@ -109,6 +109,11 @@
             "include": "gallery",
             "layout": "auto"
         },
+        "gelbooru":
+        {
+            "api-key": null,
+            "user-id": null
+        },
         "gfycat":
         {
             "format": ["mp4", "webm", "mobile", "gif"]