Should be possible to add guards

[finnsson]

Guards are methods that are run before the page navigation takes place and
that can stop the navigation from displaying a certain page.

Use the property guard: someMethod do apply the guard. The method
takes three parameters: page, route and callback. If the callback is called
the navigation takes place - otherwise it is stopped.

<div data-bind="page: {id: 'admin', guard: isLoggedIn}">
  This page is only accessible if the user is logged in.
</div>

where

isLoggedIn: function(page, route, callback) {
    if(viewModel.loggedIn()) {
        callback();
    } else {
        window.location.href = "login";
    }
}

Use cases are login, validating steps in state machines, etc.

The reason the guard takes a callback as third argument is simply because the guard might be async - accessing
a webserver for login details or asking if a valid shopping card exists etc.