Properly exert backpressure for pipeline-constructed streams.

[vqvu]

Noticed in #371 (comment).

pipeline overrides the write method but doesn't return anything. For node streams, this has the effect of disabling backpressure, since they do a check for false === write(...). For others, it may have the effect of always enabling backpressure.

We need to return the correct value. This also means supporting the drain event, which it didn't before.

My solution was to

1. construct the result stream via _(),
2. override write to return src.write(x), and
3. pipe dest to the result stream.

In the process, I refactored the logic for pipe out into a separate function that is used by both pipeline and pipe (with slightly different behavior).

This was actually more complicated than I thought, so I'd like some eyes on this besides my own.

[svozza]

I won't pretend to fully understand what's going on here because I find the messing around with write a bit confusing but this looks good. It also has the side effect of closing our oldest open PR! #101

[vqvu]

Oh, cool. I didn't realize we had an open PR for this. TBH, I'm not too fond of messing with write either, but I don't think there's any other way.