Tweak in Lang class

Changed function line() a bit, so we can pass parameters to be parsed by translation. For example:

$this->lang->line('name', 'John');

Where:

$lang['name'] = 'How is %s?';

Result:

'How is John?'
With the above notes, this should all come down to:

    return empty($args)
        ? $value
        : vsprintf($value, $args);