Function rightJoin

rightJoin(str, length?, char?): string
右边补某个字符
Parameters
- str: string | number
  字符串
- length: string | number = 2
- char: string | number = 0
Returns string
Example
```
rightJoin('1', 3, 'x'); /// '1xx'
rightJoin(0, 3, 1); /// '011'
```