不处理字符串中的非中文字符

Before：
```javascript
> pinyin('中English')
[ [ 'zhōng' ],
  [ 'E' ],
  [ 'n' ],
  [ 'g' ],
  [ 'l' ],
  [ 'i' ],
  [ 's' ],
  [ 'h' ] ]
```
After：
```javascript
> pinyin('中English')
[ [ 'zhōng' ], [ 'English' ] ]
```