Ability to utilize namespaces

I'm not sure how to do this, but it would be cool if the keys could be namespaced, and we had a way to get all of the values under a namespace..

```crystal
cache.write("user:some_special_key") { user1 }
cache.write("user:some_special_key2") { user2 }

# returns 2 CacheItem records
cache.read_all("user:*")

# remove all cache under the `user` namespace.
cache.delete_all("user:*")
```

