Add method to retrieve CacheItem rather than the value of the item

[rahulraheja]

Alternative implementation, more generic, for https://github.com/patrickmn/go-cache/pull/47

[rahulraheja]

@patrickmn Any thoughts on the same?

[rumbergd]

I have a valid use case for using this method new method - GetCacheItem. It'll let me implement custom logic based on the item expiration time... more specifically it'll be useful to implement graceful degradation using go-cache

[pablodz]

Found this fork: https://github.com/arp242/zcache