Add support for custom filter callbacks

[clue]

Add Tokenizer::addFilter() method to support adding custom filter callbacks:

$tokenizer = new Tokenizer();
$tokenizer->addFilter('ip', function ($value) {
    return filter_var($ip, FILTER_VALIDATE_IP);
});
$tokenizer->addFilter('lower', function (&$value) {
    $value = strtolower($value);
    return true;
});

$router = new Router($tokenizer);
$router->add('add <name:lower>', function ($args) { });
$router->add('--search=<address:ip>', function ($args) { });