Consolidate interface for reusable HTTP client middlewares

[h2non]

This thread is intended to provide a design discussion frame in order to consolidate a homogeneous and versatile HTTP client middleware interface that can be used in gentleman, as well as in other packages.

Ideally, the conclusion of this proposal will be introduced in gentleman@v2.

Proposal MVP:

• context.Context capable.
• Minimal, idiomatic interface, inspired by http.Handler.
• Exclusive net/http types and interfaces coupling (third-party package agnostic, even gentleman).

Proposals and ideas are very welcome.

See also related issues:

• net/context and httptrace support #17
• Evaluate HTTP tracing support #18

[h2non]

gentleman@v2 is finally out with first-class net/context support: https://github.com/h2non/gentleman