feat(AppShell): make app-shell runtime parser play nicely with SW caching

[jeffbcross]

Work with @alxhub to determine the best user experience.

• Come up with extensibility API
• Implement

[jeffbcross]

Since the parsing logic is almost done, we need to come up with a minimal API to make SW pluggable so App Shell parser can plug in seamlessly.

[jeffbcross]

Design is fleshed out, now a matter of implementing. Still 2-3 days left of work.