流这一章非常的重要，官方网店将它分成2个部分讲解：
第一部分是介绍API,教你怎么用
第二部分是教你怎么通过API实现你自己的流
Types of Streams#
Node.js 给出了最基本的4中流类型:
- Readable - streams from which data can be read (for example
fs.createReadStream()). - Writable - streams to which data can be written (for example
fs.createWriteStream()). - Duplex - streams that are both Readable and Writable (for example
net.Socket). - Transform - Duplex streams that can modify or transform the data as it is written and read (for example
zlib.createDeflate()).
Buffering
Writable and Readable streams 都会在内部存储data，并且通过writable._writableState.getBuffer() or readable._readableState.buffer 来使用它。
当然，data的buffer量 是依靠 highWaterMark 来决定的（highWaterMark 会传入流的构造函数）。对于普通的流，highWaterMark 就是指定的字节总数，对于object mode下的 highWaterMark，highWaterMark 指定了对象的总数。
从Readable streams 里来的缓