Simple strongly typed middleware-style router
Allowing user to mutate request directly (imperative code) is almost impossible to type correctly. Also, we cannot check at compile-time if user follows the contract that he defined (if he defines it at all). Instead, we can make middlewares declarative: they will return mutations that we can check at compile-time against their interfaces and then apply to request under the hood.
TODO: Better README: npm link, description, examples, installation instruction, docs