Scoped pure view observables using `vars`

[finnsson]

<!-- x and y are now available as observables, but only in the view (not the view-model) -->         
<div data-bind="page: {id: 'foo', vars: ['x','y']}">                                                 
  <span data-bind="text: x"></span>                                                                  
  <span data-bind="text: x"></span>                                                                  
</div>                                                                                               

This is useful for effects that has nothing to do with the view-model.