Feature grouped routes

This enables grouped routes, which can be very handy for long or repetitive routing tables. Here's an example:

```php
$app['route'] = [
    '@/foo' => [
        '/bar' => 'Foo::bar',  // This is really '/foo/bar'
        ...
    ],
    '@/baz' => [
        '/qux' => 'Baz::qux', // This is '/baz/qux'
    ],
];
```