Allow replacing a default typereader globally

src/Discord.Net.Commands/CommandService.cs

@@ -263,6 +263,14 @@ namespace Discord.Commands
             }
             return null;
         }
+        public void ReplaceDefaultTypereader<T>(TypeReader reader)
+        {
+            var key = typeof(T);
+            if (_defaultTypeReaders.ContainsKey(key))
+            {
+                _defaultTypeReaders.AddOrUpdate(key, k => throw new Exception("This shouldn't happen?"), (k, v) => reader);
+            }
+        }
 
         //Execution
         public SearchResult Search(ICommandContext context, int argPos)