`withOnShow` should lazy bind a new view model to the page

```
<div data-bind="page: {id: 'user', withOnShow: someMethod('someMethod')}"></div>
```

`someMethod` must return a function that takes a callback that takes a view model.

E.g.

```
function requireVM(module) {
  return function(callback) {
    require([module], function(mod) {
      callback(mod.getVM());
    });
  };
}
```
