This is both a great post and an effective ad - I've been looking for a lighter-weight Postman alternative (and HTTPie, while nice, is no substitute for a graphical UI for such a thing). Will check HTTP Toolkit out!
Do try out HTTP Toolkit and let me know what you think, but it's not a general purpose HTTP client like Postman or HTTPie. It's actually an HTTP debugger, more like Fiddler/Charles/mitmproxy, for debugging & testing. A convenient HTTP client is definitely planned as part of that eventually, but not today.