Expose addCorsHeaders

[jacwright]

This should allow cors headers to be added at the end of a call.

Really, Response middleware (like ThrowableRouter) should be made stackable so we can have CorsRouter.

Maybe something like this?

const router = CorsRouter(corsOptions, ThrowableRouter({ stack: true }, Router({ base: '/' })))

Or perhaps:

const router = Router();
...
  event.respondWith(isThrowable(router.handle(event.request)))

[kwhitley]

Really, Response middleware (like ThrowableRouter) should be made stackable so we can have CorsRouter.

Agreed!