[FEATURE] Strongly-typed user data (or: implementing generic traits for Cursive based on user data)

Hi @gyscos! I would like thank you for this nice library and ask you a question regarding extending `cursive` for more structured data/state management.

It seems that user data is typed as `Any`, is that right? Could it be possible to support some sort of strong typing? That would allow one to trivially implement new traits for `Cursive` by leveraging the internal data (by delegating methods to the internal object, **I'm specifically trying to implement the Redux pattern** as a trait). I'm not sure of this is possible without the compiler knowing the data type.

One could always default to a boxed `Any` or something like that, so that wouldn't require breaking things. What do you think about that?