middleware support

[Ppjet6]

I would like to be able to hook things on the model, that could do other things, whatever they want. I have a logger in mind, or something that would allow me to play with the state (rewind/forward), analyse it etc.

I don't think these things should be in cedar itself, but in other crates, only the middleware interface would be required.

To be a bit more specific, see how redux does it. I don't have a particular implementation in mind though, that's just one I know of.