Added default config for domain substitution

twoot.py

@@ -445,6 +445,12 @@ def main(argv):
     # 3. The value provided on the command line
 
     # Default options
+    substitution = {
+        'twitter': [],
+        'youtube': [],
+        'reddit': [],
+    }
+
     options = {
         'upload_videos': False,
         'post_reply_to': False,
@@ -454,6 +460,7 @@ def main(argv):
         'tweet_max_age': float(1),
         'tweet_delay': float(0),
         'toot_cap': int(0),
+        'substitution': substitution,
     }
 
     # Default toml