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',
});
```
I think you should be able to override a an option set on Router level with "undefined" on i.e. route level.
```js
Router.configure({'authenticate.option': 'myOptionValue'});

Router.route(function() {
  ...
  authenticate: {
    option: undefined
  }
  ...
});
```
Which is only possible if you check for the existenze of the key and only get the value if the key exists.

However I could look into having a funktion that returns an object with the value if the key exists instead of a hasPath/getPath approach