Accept cache parameter on Memoize constructor

I have a use case where I need to memoize using a DatabaseCache backend.  

It would be great if `Memoizer` constructor accepted a cache parameter, allowing the developer to specify a custom cache like: `db_memoizer = Memoizer(cache=get_cache('db_cache'))`

Also, it would be interesting to add a `cache` parameter to `@memoize` decorator. What do you think?