using `compose` for returning 3 and 4 arity middlewares

Hi @blakeembrey

I find your package and it's just what I was looking for. I really like this package because it can helps to split my middlewares into little pieces and compose them. My congrats!

However I would like to understand why is `errors` function needed. I think it's possible to have just one `compose` function:

- If the first middleware has less than 4 arguments, then it returns a 3-arity middleware.
- If the first middleware has 4 arguments, then it returns a 4-arity middleware.

I'm basing this proposal on that usual composition returns a function with the same arity than first called function.

What do you think?