write OAuth token to cache by default (#616)

docs/gallery-dl.conf

@@ -30,7 +30,6 @@
         },
         "deviantart":
         {
-            "refresh-token": null,
             "extra": false,
             "flat": true,
             "folders": false,
@@ -51,8 +50,6 @@
         },
         "flickr":
         {
-            "access-token": null,
-            "access-token-secret": null,
             "videos": true,
             "size-max": null
         },
@@ -96,6 +93,7 @@
         "oauth":
         {
             "browser": true,
+            "cache": true,
             "port": 6414
         },
         "pixiv":
@@ -120,7 +118,6 @@
         },
         "reddit":
         {
-            "refresh-token": null,
             "comments": 0,
             "morecomments": false,
             "date-min": 0,