Support Spec namespace

[kkharji]

While working on gathering requirement for #9 it became apparent that duct.reitit/routes would become hard to read easily.

To solve this potential unwanted complexity in configurations, duct.core/spec-ns should be supported the same way duct.core/middleware-ns and duct.core/handler is.

Users then can define spec for requests and responses like <project-ns>.<spec-ns>/login-user, <project-ns>.<spec-ns>.user/login and then either reference them with a keyword after defining them in registry or reference the symbol directly