Allow the public access getCacheFile method.

The pre-caching functionality requires fairly complicated implementation which may also under the help by other libraries. I think it may distract the core functionality on this library. In the case it seems better to allow the public access of the getCacheFile method (Method to get the cache file path if I don't get wrong.) which allows custom way to implement the pre-caching function according to the use case (e.g. Need reactive way to pre-cache the file, parallel download of the files etc.).