Do not authenticate CORS preflight OPTIONS requests by default

You can use the ValidatorInterface to implement any other arbitary rules
of allowing request to pass through even though it normally should be
authenticated.

$app->add(new \Slim\Middleware\HttpBasicAuthentication([
    "path" => "/admin",
    "realm" => "Protected",
    "validator" => new FooValidator()
]));
Prepare for 0.8.1