The Pushstate constructor should work independent of can-route

[justinbmeyer]

I think that Pushstate should work independent of can-route, but it does not. For example, a codepen like this does not work:

https://codepen.io/justinbmeyer/pen/MzGyPb?editors=1011

This is because:

1. Pushstate calls into can-route to know the root (which is going to be #!)
2. When a link is clicked on, the routing rules are checked: if (route.rule(url) !== undefined) {

#1 can be solved by letting people pass the "root" into new RoutePushstate().

#2 is tricker. We could allow people to pass in if the "url" should be handled.

new RoutePushstate({
  root: "/",
  usePushstate(){
    return true; // All links will use pushstate
  }
})

I think we should enable this sort of flexibility now. It will make issues like canjs/canjs#4448 easier to tackle.

For backwards compatibility, we will just make the constructor default to:

new RoutePushstate({
  root: function(){
    bindingProxy.call("root")
  },
  usePushstate(){
    return true; // All links will use pushstate
  }
})

The other place it is used is when setting the value of the observable (obs.set("/some/url")).

Currently, set(){} uses route.deparam() to diff which keys are changing and optionally call replaceState instead of pushState.

Do we need this functionality? Should we expect people to call these themselves? We could have additional configuration like:

new RoutePushstate({
  historyMethod(path) {
    return CHECK_PATH ? "pushState" : "replaceState"
  }
})