Programming in Haskell -- adding topics and thinking about effects

I'm ready to add "topics" to showtime. The Storage module is now entirely independent from the Host and Participants so I can use it to store a new set of data. This means I can add commands to add, remove, and list topics.

In order to do this I'll need to add some extra configuration for the filepath to store topics, so this episode might turn into a foray in `ReaderT IO` or maybe even effectful.