Nested/Sub Blueprint

Being able to have nested/sub Blueprint is a crucial feature to have when building out API. This way we could split code into multiple different files, modular approach to API and easy to maintain.

The below code is already in used in many of my projects (hobby and work). The purpose of this github issue is to discuss whether there are problem with my implementation, or issues, so on. Happy to put a PR forward.

Gist: https://gist.github.com/jimmyruann/9e3214780739a130b7a22fc5ca9d183b