fs.createWriteStream(path[, options])
options 还可以包括 start 选项，以允许在文件开头的某个位置写入数据，允许的值在 [0, Number.MAX_SAFE_INTEGER] 范围内。
修改文件而不是替换它可能需要将 flags 选项设置为 r+ 而不是默认的 w。
encoding 可以是 <Buffer> 接受的任何一种。
如果将 autoClose 设置为 true（默认行为），则在 'error' 或 'finish' 时文件描述符将自动关闭。
如果 autoClose 为 false，则即使出现错误，文件描述符也不会关闭。
关闭它并确保没有文件描述符泄漏是应用程序的责任。
默认情况下，流将在销毁后触发 'close' 事件。
将 emitClose 选项设置为 false 以更改此行为。
通过提供 fs 选项，可以覆盖 open、write、writev 和 close 的相应 fs 实现。
在没有 writev() 的情况下覆盖 write() 会降低性能，因为某些优化 (_writev()) 将被禁用。
当提供了 fs 选项时，则至少需要覆盖 write 和 writev 之一。
如果没有提供 fd 选项，则还需要覆盖 open。
如果 autoClose 是 true，则还需要覆盖 close。
与 <fs.ReadStream> 一样，如果指定了 fd，则 <fs.WriteStream> 将忽略 path 参数并使用指定的文件描述符。
这意味着不会触发 'open' 事件。
fd 应该是阻塞的；非阻塞 fd 应该传给 <net.Socket>。
如果 options 是字符串，则它指定编码。
options may also include a start option to allow writing data at some
position past the beginning of the file, allowed values are in the
[0, Number.MAX_SAFE_INTEGER] range. Modifying a file rather than
replacing it may require the flags option to be set to r+ rather than the
default w. The encoding can be any one of those accepted by <Buffer>.
If autoClose is set to true (default behavior) on 'error' or 'finish'
the file descriptor will be closed automatically. If autoClose is false,
then the file descriptor won't be closed, even if there's an error.
It is the application's responsibility to close it and make sure there's no
file descriptor leak.
By default, the stream will emit a 'close' event after it has been
destroyed. Set the emitClose option to false to change this behavior.
By providing the fs option it is possible to override the corresponding fs
implementations for open, write, writev, and close. Overriding write()
without writev() can reduce performance as some optimizations (_writev())
will be disabled. When providing the fs option, overrides for at least one of
write and writev are required. If no fd option is supplied, an override
for open is also required. If autoClose is true, an override for close
is also required.
Like <fs.ReadStream>, if fd is specified, <fs.WriteStream> will ignore the
path argument and will use the specified file descriptor. This means that no
'open' event will be emitted. fd should be blocking; non-blocking fds
should be passed to <net.Socket>.
If options is a string, then it specifies the encoding.