Make num_links=0 in pagination library possible to configure

In pagination library, num_links = 0 wasn't supported so far.
So I modified system/libraries/Pagination.php and made num_links = 0 configuration available.
Revert the changes *inside* this codeblock and change this line to:

    if ($this->first_link && $this->cur_page > ($this->num_links + 1 + ! $this->num_links))