Add filter to limit posts on zone

Added filter 'zoninator_post_count_limit' to limit the number of posts that can be added to a zone. Pass the zone_id to determine which zone you want to limit the number of posts for. A filter could look something like this: 
    add_filter('zoninator_post_count_limit', 'limit_number_of_posts', 10, 1);
    
    function limit_number_of_posts($zone_id) {
        switch($zone_id) {
            case '2':
                return 1;
            break;
        }
    }

0 allows any number of posts to be added to a zone. Greater than 0 means that that many posts can be added to a zone. Didn't add the capability to not add posts to a zone, but that could be implemented.