Proof of concept for terminal forwarding

[asakusuma]

This PR adds an option that allows declaring a route to be an invalid terminal state, while also declaring what route should actually be visited if someone tries to visit as the terminal route.

TODO:

1. Figure out perf implications
2. Decide on exact API for the current "terminal" option in the dsl
3. Support API for applying the terminal option to the parent instead of the index?

[nathanhammond]

Spaces here.

[nathanhammond]

Writing some documentation:

---

The use-case for specifying a terminal route is _not_ to prevent us from having to create a route whose content is simply:

beforeModel() {
  this.replaceWith('non-terminal.sibling');
}

The goal is instead to make sure that redirects of this nature which are not dynamic can easily be identified and accounted for in other tools such as ember-prefetch.

[nathanhammond]

Looking at ember-prefetch we need some way to make sure that this has settled by the time we trigger willTransition. Flow as it stands:

1. Overridden willTransition inside of your PR.
2. Ember.Router.willTransition, which triggers the willTransition event.
3. ember-prefetch's willTransition event handler.

Which means that we can't prevent things like ember-prefetch from running to completion on the original transition object. We need some way to capture this before the willTransition is fired/occurs, so that the "first" transition is to the correct location. (This scenario is almost universally going to be triggered by directly visiting some URL as there shouldn't be any links to non-terminal routes.)

[nathanhammond]

I think we might be able to get away with manually passing forward the new transition from line 22 into _super as long as we're 100% certain that this is the first reopen.

/cc @nickiaconis