Add a formatter to the registry
        if key_specific is provided, this formatter will only be used for some
        specific exports

def add_formatter(self, sqla_col_type, formatter, key_specific=None):
        """
        Add a formatter to the registry
        if key_specific is provided, this formatter will only be used for some
        specific exports
        """
        self.add_item(sqla_col_type, formatter, key_specific)