[ENH] Add Cache Size Utility Function

Users have no easy way to check how much disk space the OpenML cache is using. The cache can grow large with many datasets. The solution would be to add a utility function to calculate the current cache size. 

Much of the relevant code is in `openml/config.py` and `openml/utils.py`. We could put the function into `openml/utils.py` and expose it to the public API from there.