add FileToSlice function

Function FileToSlice read a file and return all its data into a 3d slice,and treat all cell as a string type.This function simplify my game data excel importer code greatly.

```
rawArray, err := xlsx. FileToSlice("/home/tealeg/foo.xlsx")
if err != nil {
    panic(err)
}
title:=rawArray[0][0]
...    
```

I add FileToSlice to this project to help other simplify their code.

I had write this function on 9 apr. in https://github.com/bronze1man/kmg/blob/master/encoding/kmgExcel/XlsxFile2Array.go#L13