support external piping

[mafintosh]

Right now there doesn't seem to be a way to do

var req = get.post(...)

pump(coolStream, req)

Would be great to support that :)

[bendrucker]

I added support for using readables as the body in #30. It's obviously not as elegant as piping to a stream returned by simple-get but that seemed like it would clash too much with the existing interface. At a bare minimum, streaming use can't call req.end. I think this would require a different method/option indicating that the user wants a writable stream.

[mafintosh]

@bendrucker possible solution is to nextTick the req.end() and if req.on('pipe') fires inbetween cancel it.

[bendrucker]

Mmm good point!