Introduce exact matching

The use case:
I have a POST '/users/' method which should be anonymous. However POST '/users/123' should NOT.

Solution: introduce "exact_match" options to the firewall i.e.
```php
$kernel = (new \Stack\Builder())
            ->push('Dflydev\Stack\BasicAuthentication', [
                'firewall' => [
                    ['path' => '/'],
                    ['path' => '/users/', 'exact_match' => true, 'method' => 'POST', 'anonymous' => true],
                ],
                'authenticator' => $auth,
                'realm' => '...',
            ])
            ->resolve($app);
```

Would you accept the PR?