Allow use of ReferenceProviders to pull in required/supporting assets for Pre-Cache and Fallback resources

[davidjgonzalez]

Update: perhaps this is not a great idea since most paths on web pages to images should not be directly to the referenced resource; but rather through some proxy (image component, named image transform, etc.) which would not be fixed by this.

Fallback and Pre-Cache resources (defined in the Cloud Service and exposed via the service-worker.json) are typically paths to pages. However, if these pages require supporting images/etc., they are not immediately loaded, resulting in "broken" experiences on these precached pages.

We should look at hooking in the reference provider API and including referenced paths in the pre-cache/fallback lists.