add add_alias method to asset filters

miyu_bot/bot/master_asset_manager.py

@@ -55,7 +55,10 @@ class MasterFilter:
             self.unrestricted_filter[name] = master
         if aliases:
             for alias, mid in aliases.items():
-                master = masters[mid]
+                self.add_alias(alias, mid)
+
+    def add_alias(self, alias, master_id):
+        master = self.masters[master_id]
         alias = romanize(alias)
         self.default_filter[alias] = master
         self.unrestricted_filter[alias] = master