Reconsider context support

[dgl]

Hi, I saw #103, but I think it is worth revisiting that now.

It would be useful to have context support for timeouts and tracing.

There's a comment on #103 that this won't mix with io.Reader, but see for example the newer google storage API at https://godoc.org/cloud.google.com/go/storage which uses that interface and supports context on top of it.

I think maybe using a model a bit like http.Request does now of having a .WithContext could work with minimal API changes (and/or add *Context versions of functions).