[reddit] fix submission-ID parsing (#104)

Uppercase characters caused a ValueError exception

docs/gallery-dl.conf

@@ -85,7 +85,7 @@
             "date-max": 253402210800,
             "date-format": "%Y-%m-%dT%H:%M:%S",
             "id-min": "0",
-            "id-max": "ZIK0ZJ",
+            "id-max": "zik0zj",
             "recursion": 0
         },
         "sankaku":

gallery_dl/extractor/reddit.py

@@ -304,7 +304,7 @@ class RedditAPI():
 
     def _parse_id(self, key, default):
         sid = self.extractor.config(key)
-        return self._decode(sid.rpartition("_")[2]) if sid else default
+        return self._decode(sid.rpartition("_")[2].lower()) if sid else default
 
     @staticmethod
     def _decode(sid):