[reddit] enable user authentication with OAuth2 (#15)

Call '$ gallery-dl oauth:reddit' to get a refresh_token
for your account.

docs/gallery-dl.conf

@@ -50,6 +50,14 @@
         "gelbooru":
         {
             "filename": "{category}_{id:>07}_{md5}.{extension}"
+        },
+        "reddit":
+        {
+            "refresh-token": null,
+            "comments": 500,
+            "date-min": 0,
+            "date-max": 253402210800,
+            "recursion": 0
         }
     }
 }