Add "has" method

[eight04]

I want to check if a resource has already been cached. Maybe we can have a has method or a getMeta method that only returns the metadata without the binary file? Also, we need a method to refresh a cache i.e. to update the expire tag of the resource.

Other questions:

1. What will happen when the size limit is reached?
2. What will happen when the count limit is reached?
3. Is it possible to use unlimited size and count?

[eight04]

After working on #19. I found that what I really need is the ability to save metadata (file size, name, etc) along with the binary. For example:

// getting the metadata
try {
  return await cache.getMeta(key);
} catch (err) {}
const {resource, meta} = await fetchResource(key);
await cache.set(key, resource);
await cache.setMeta(key, meta);
// it will be better to set both the resource and the metadata at once
// e.g. cache.add({key, value: resource, meta})
return meta;