Allow automatic re-fetching of common requests

I think it would be a great feature to enable auto re-caching on a schedule - so clients almost always get a cache hit.
Something like
```ts
cache.fetch(..., { automaticallyRefetchUntil: Date.now() + 30 * DAYS });
```
That would call it with the same arguments a ~minute before it expires until the specified date - so you get cache hits without infinitely re-fetching

Inspired by #6