[derpibooru] use the "Everything" filter by default (#1243)

when neither 'api-key' nor 'filter' are set

docs/configuration.rst

@@ -713,8 +713,6 @@ extractor.derpibooru.filter
 Type
     ``integer``
 Default
-    ``null``
-Example
     ``56027`` (`Everything <https://derpibooru.org/filters/56027>`_ filter)
 Description
     The content filter ID to use.

gallery_dl/extractor/derpibooru.py

@@ -46,6 +46,8 @@ class DerpibooruExtractor(BooruExtractor):
         filter_id = self.config("filter")
         if filter_id:
             params["filter_id"] = filter_id
+        elif not api_key:
+            params["filter_id"] = "56027"  # "Everything" filter
 
         while True:
             data = self.request(url, params=params).json()