Make extend() return manager instance.

Related to #2608. Turning code like this:

	$this->app->extend('auth', function($auth, $app)
	{
		$auth->extend('fluxbb1', function($app)
		{
			$connector = $app['fluxbb1.db.connector'];
			$provider = new UserProvider($connector->connection());

			return new Guard($provider, $app['fluxbb1.cookie.storage']);
		});
		return $auth;
	});

Into this:

	$this->app->extend('auth', function($auth, $app)
	{
		return $auth->extend('fluxbb1', function($app)
		{
			$connector = $app['fluxbb1.db.connector'];
			$provider = new UserProvider($connector->connection());

			return new Guard($provider, $app['fluxbb1.cookie.storage']);
		});
	});

Resulting in less boilerplate when extending container bindings.