Slice: find nothing case (#11)

The current behavior can result into wired edge cases.

**Current behavior**

if find was unsuccessfully then it will return the first element of the
slice

**Desired behavior**

if find was unsuccessfully then it should return negative ok and a none
value

README_zh-CN.md

@@ -395,7 +395,7 @@ func Difference(slice1, slice2 interface{}) interface{} //返回
 func DeleteByIndex(slice interface{}, start int, end ...int) (interface{}, error) //删除切片中start到end位置的值
 func Drop(slice interface{}, n int) interface{} //创建一个新切片，当n大于0时删除原切片前n个元素，当n小于0时删除原切片后n个元素
 func Every(slice, function interface{}) bool //slice中所有元素都符合函数条件时返回true, 否则返回false. 函数签名：func(index int, value interface{}) bool
-func Find(slice, function interface{}) interface{} //查找slice中第一个符合条件的元素，函数签名：func(index int, value interface{}) bool
+func Find(slice, function interface{}) (interface{}, bool)//查找slice中第一个符合条件的元素，函数签名：func(index int, value interface{}) bool
 func Filter(slice, function interface{}) interface{} //过滤slice, 函数签名：func(index int, value interface{}) bool
 func FlattenDeep(slice interface{}) interface{} //将slice递归为一维切片。
 func IntSlice(slice interface{}) ([]int, error) //转成int切片