support TypeScript Generics #19
Description
See chimurai/http-proxy-middleware#1017 (comment)
Though http-proxy is not in typescript, there is https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/http-proxy/index.d.ts which provides typings. And in some ways these typings are better than what we have here in http-proxy-3. Fix that, in particular, with compatible generic support.