排序差异流
给定两个已排序的流，将仅发出不匹配的对
用法
var diffStream = require（'sorted-diff-stream'）
返回一个可用于创建新差异的函数
有关API示例，请参见test.js
例子
var diffStream = require ( 'sorted-diff-stream' )
var through = require ( 'through2' )
var a = through ( )
var b = through ( )
var diffs = diffStream ( a , b , isEqual , compare )
// this is the default `isEqual`, you dont have to pass one above
function isEqual ( a , b , cb ) {
cb
