Add support for CURLFORM_STREAM

[mitsuhiko]

Currently it's impossible to upload multiform files that are dynamically read from something. In raw curl one seems to use CURLFORM_STREAM paired with CURLOPT_READFUNCTION for this. Would be great if this could be supported somehow.

[alexcrichton]

Seems plausible to me!

Are you familiar with how this works in C? I'm curious to clarify how CURLOPT_READFUNCTION and CURLFORM_STREAM interact if you set both

[mitsuhiko]

The form thing has a data parameter which is pssed to readfunction as last parameter and as far as i remember overwrites what is passed there normally by default.

[alexcrichton]

Ok! I think I've got an implementation at #164, mind giving it a spin and see if it works out for you?

[JimFuller-RedHat]

does this work now ? can the PR be merged ?