Support for `expires_at` option in ActiveSupport::Cache::Store

[Jose Camacho]

Add `expires_at` to expire cache similar to `expire_in`

```

  <% cache object, expires_at: object.created_at + 1.month do %>

    <!-- something -->

  <% end %>

```

I'm not sure if this is possible or if it breaks something else, but I can't find it anything about this in the cache guides.

Thanks

[DHH]

Sounds good to me!