Add support for long streams of files

As reported by @jgoz , gulp tasks using `taskFromStreams` would not complete when `gulp.src` found more than 29 files. The problem seems to be related to buffering and back-pressure handling logic in node's stream2 implementation. By adding a writable "sink" stream to the end of pipeline, gulp tasks now complete as expected.
Can you add one more comment explaining why we need the writable sink at the end of the piping sequence?