Add functionality to restrict the disk space used by the Cache

Hello @epwalsh, thank you for this project, it's very useful!
One issue that I have is that the cache sometimes takes up all of the disk space, for when I'm doing an operation that moved a lot of data around.
A solution for that would be to implement a maximum size which the cache can occupy on disk and a strategy to remove entries.

If you have some specific idea or direction on how this could be implemented I'd be happy to do it and write tests for it.
Cheers