update default gallery-dl.conf

add (empty) entries for
- deviantart.client-id and deviantart.client-secret
- pixiv.refresh-token

docs/gallery-dl.conf

@@ -59,6 +59,8 @@
         },
         "deviantart":
         {
+            "client-id": null,
+            "client-secret": null,
             "extra": false,
             "flat": true,
             "folders": false,
@@ -183,6 +185,7 @@
         },
         "pixiv":
         {
+            "refresh-token": null,
             "avatar": false,
             "tags": "japanese",
             "ugoira": true