context.Context support

[abhinav]

Overview

Go client libraries typically accept a context.Context as the first
argument in all outgoing function calls. The context carries deadline and
other information across API boundaries. See also https://blog.golang.org/context.

Popular examples are,

• go-github
• Google Cloud
• AWS

Context also allows orchestrating cancelation, especially with concurrent
operations. Support for this is already built into http.Client.

Implementation

All methods that make an outgoing request would have to accept a
context.Context as their first argument and plumb it over to http.Client
with req.WithContext(ctx), likely in buildRequest or prepareRequest.

Because this will involve a breaking change to the API, it should probably be
bundled with any other breaking changes you have planned.

[thebaer]

Great input! If anyone wants to jump on this for the v2 release of the library, please feel free to do so now. It'll be a good time to bring in this breaking change.