Add basic support for modes: none, new_episodes, once

We also needed support for the `none` mode, so I created a very simple implementation. 

Please let me know if you have any feedback or if you would like me to create some automated tests. I wanted to check to see if the implementation was acceptable first.
Those two classes seem to share partly the same logic, I'm pretty short on time but it would be best to extract that to a abstract base `Storage` class.