Support cache value loading on cache miss

Hi,

Thanks for this great cache implementation. Pretty simple but still better than many other js caches...

It would be nice to have more simple options, like we have in Guava for Java, to be able to populate the cache with the required data on cache miss (Guava LoadingCache + CacheLoader).
I mean loading synchronous but expensive to compute data, nothing to do with promises (not my usecase but it would be nice too)

See http://eclipsesource.com/blogs/2012/07/09/building-lightweight-in-memory-caches-with-google-guava-no-more-putifabsent/