Provide a param observable instead of value

An example from readme:

``` js
const routes = {
  '/:id': id => sources => YourComponent({props$: Observable.of({id}), ...sources})
}
```

Here, whenever the `id` is changed, a new `YourComponent` is created. It would be nicer to have something like

``` js
const routes = {
  '/:id': param$ => sources => YourComponent({param$, ...sources})
}
```

Where `param$` emits values of form `{id}`
