Ability to insert if not exists

[vortechron]

It's good to have the ability to provide optional flag --replace / --existable so that the seeder can insert the data if not exist yet, or maybe replace the data if already exists without the id key

example:

        $dataType = $this->dataType('slug', 'menus');
        if (!$dataType->exists) {
            $dataType->fill([
                'name'                  => 'menus',
                'display_name_singular' => __('voyager::seeders.data_types.menu.singular'),
                'display_name_plural'   => __('voyager::seeders.data_types.menu.plural'),
                'icon'                  => 'voyager-list',
                'model_name'            => 'TCG\\Voyager\\Models\\Menu',
                'controller'            => '',
                'generate_permissions'  => 1,
                'description'           => '',
            ])->save();
        }