add command `brew cask cleanup`

It came to my attention that Homebrew's `brew cleanup` is not reliably cleaning up cached downloads from homebrew-cask.

There is no easy way to solve that for old downloads (users must figure out that the cache directory is huge and clean it out manually).

This PR addresses downloads going forward, by making a tracker symlink to all files downloaded by homebrew-cask, and adding a new command `brew cask cleanup` which chases the tracker symlinks and deletes cached downloads older than 7 days.