Basic transform stream
Got default close propagation working
Implement flushing
Handle errors thrown by transform
Fix tests in light of bda8ab
Make the transform streams actually propagate backpressure.

The previous approach was too naive. This one correctly waits to do a transform until backpressure has eased up.
Update tests in light of 6a84b5.

Transform streams no longer add queues in which backpressure can be delayed.
Handle errors thrown by flush.
Merge branch 'transform'
Add a pointer to TransformStream in the readme.