PSR-16 Compatibility

[HeathNaylor]

I am working on creating a general wrapper for various feature flag libraries to act as an abstraction point. Currently this proves somewhat difficult to do due to the lack of PSR extension in the interfaces of Rollout. An example of this can be found in the StorageInterface. If the interface was removed and instead all storage classes extended the SimpleCache PSR interface (https://github.com/php-fig/simple-cache/blob/master/src/CacheInterface.php), we would have all the same functionality and then some.

Is Rollout intended to only be a clone of the Ruby version, or is the room for bringing in more modern PSR interfaces?