Add ability to override Dispose() in the derived class (#1256)

Dapper.Rainbow/Database.cs

@@ -464,7 +464,7 @@ private bool TableExists(string name)
         /// <summary>
         /// Disposes the current database, rolling back current transactions.
         /// </summary>
-        public void Dispose()
+        public virtual void Dispose()
         {
             if (_connection.State != ConnectionState.Closed)
             {