Function leftJoin

leftJoin(str, length?, char?): string
左边补某个字符
Parameters
- str: string | number
  字符串
- length: string | number = 2
- char: string | number = 0
Returns string
Example
```
leftJoin('1', 3, '0'); /// '001'
leftJoin(0, 3, 1); /// '110'
```