public function add($columns, $label = null)
    {

        if (!is_array($columns)) {
            $columns = [$columns => $label];
        }

        foreach ($columns as $column => $label) {
            if (is_null($label)) {
                $label = str_replace(['_', '-'], ' ', ucfirst($column));
            }

            $this->add[$column] = $label;
        }

        return $this;
    }