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.
Maybe adding an assertion on the mode would help people not making typos. I usually add a property specifying the possible options (see for `$availableStorages`) and then test the parameter in the setter, similar to  https://github.com/johnmadrak-wobo/php-vcr/blob/request_blocking_support/src/VCR/Configuration.php#L299.