Support private service container

[torreytsui]

Objective

Should we offer the same functionality as self::$container in Symfony 4.1 for behat context?

https://symfony.com/blog/new-in-symfony-4-1-simpler-service-testing

Related: #139

Proposed approach

Maybe, we just need to document how we can resolve it?

class FooContext implements Context, KernelAwareContext {
    public function doSomething(...) {
        $container = $this->kernel->getContainer();

        // Same as the self::$container, which can access private services
        $privateServiceContainer = $container->has('test.service_container') ? $container->get('test.service_container') : $container;
    }
}

For one more step forward, would it be more developer friendly if an interface is offered and injects the container automatically? (e.g., PrivateServiceContainerAwareContext)