Add a collection ID to the wp_cache key

- Create one cache for each shortcode (aesop_collectio)n using the collection ID.

BUG: When you use the shortcode twice:

´´´
[aesop_collection title="xx" collection="1" columns="4"]
[aesop_collection title="xx" collection="2" columns="4"]
´´´

The posts for the first collection are used again, this is because the wp_cache.