Add support for dot notation in lookupOption

Opening this in favor of #959.

```js
Router.onBeforeAction(function() {
  loginTemplate = this.lookupOption('authenticate.loginTemplate');
});

Router.route('/', {
  authenticate: {
    loginTemplate: 'signIn'
  },
  name: 'home',
});
```
@Multiply @tmeasday any thoughts on this?

Should you be able to `unset` an options set in Router level in Route/RouteController level?

I can see a need for this in my plugin but I don't know if my case might be the "exception" from the general usage