Add public method to request the disk cache to cleanup

Applications may need to explicitly request a disk cache cleanup in some circumstances.

For example in our application we detect if the device hits an exception due to being very low on disk space, and if it is we reduce the size of our disk cache and immediately tell it to discard some files. Doing that our application will be functional until there is no more space to release from our internal directories.

(We did originally create a PR for this, but we accidentally closed it: https://github.com/JakeWharton/DiskLruCache/pull/50)