Returning channels and streams

[mcollina]

We need a way from a Seneca service to return a binary stream and a channel.

Currently, it's really easy to add a binary stream inside the seneca response. However, the same is not true for channels, as you need to create those from another channel. Currently I'm trying to figure out if it make sense if streams in objectMode should be auto-piped as channels, but it has its own issues: GraftJS/jschan#26

In graft, we pass the channel and the session in the message as non-enumerable properties. You might want to depend on graft directly to just reuse that code (it's also simpler to use).