[wikimedia] implement config lookup for fandom/wikigg sites (#7283)

{
    "extractor": {
        "fandom": {
            "filename": "..."
        }
    }
}

test/test_job.py

@@ -214,6 +214,7 @@ Request interval (default):
     def test_base_category(self):
         extr = TestExtractor.from_url("test:")
         extr.basecategory = "test_basecategory"
+        extr.basesubcategory = "test_basesubcategory"
 
         self.assertEqual(self._capture_stdout(extr), """\
 Category / Subcategory / Basecategory