[#2] Added ability to specify custom layout

Works exactly like requesting in issue #2.

return array(
    'router' => array(
        'routes' => array(
            'about' => array(
                'type' => 'Literal',
                'options' => array(
                    'route' => '/about',
                    'defaults' => array(
                        'controller' => 'PhlySimplePage\Controller\Page',
                        'template'   => 'application/pages/about',
                        'layout' => 'layout/static-pages'
                    ),
                ),
            ),
        ),
    ),
);