[FEATURE ember-application-engines] Enable loading and error substates for mounts

[trentmwillis]

Reference: ember-engines/ember-engines#103

One open question, how shold substates work when used in conjunction with resetNamespace (with either mount or route)? In other words if I have:

Router.map(function() {
  this.route('news', function() {
    this.{mount|route}('blog', { resetNamespace: true });
  });
});

Should the path to the substate then be: blog_loading or news.blog_loading? I feel like it should be blog_loading but in the current checks the namespace is always applied (see here), so it will look for news.blog_loading even if resetNamespace is applied. Seems like maybe this a bug? If so, I can fix that while I'm in here.

cc @rwjblue @dgeb @nathanhammond

[rwjblue]

@trentmwillis - Yes, I agree that this is a bug.

[dgeb]

Yep, I agree that it's a bug as well. It's not specific to engines, so let's address this in a separate PR.

@trentmwillis given that, are you comfortable removing the "WIP" label so we can land this?

[trentmwillis]

I need to make some minor adjustments, but will do that this evening.

[trentmwillis]

@dgeb updated, should be good to merge once tests finish.