Pipe this stream to another.

def pipe(self, transformer):
		"""Pipe this stream to another."""
		if self.next:
			return

		stream = Stream()
		self.next = stream
		stream.prev = self

		self.transformer = transformer
		transformer.stream = self
		transformer.piped()

		for file in self.files:
			future = asyncio.ensure_future(self.transformer.transform(file))
			future.add_done_callback(self.handle_transform)	

		self.onpiped.set_result(None)
		self.flush_if_ended()

		return stream