support utf-8 code in case of Chinese comment or others

The program will go wrong because of Chinese comment in code. It doesn't work well for none ASCII encoding.

The error is as below.

```
Wring to binary-tree-postorder-traversal/Solution.2144703.cpp
Traceback (most recent call last):
  File "./leetcode_downloader.py", line 105, in <module>
    f.write(code)
UnicodeEncodeError: 'ascii' codec can't encode characters in position 320-328: ordinal not in range(128)
```

I add some simple codes to support utf-8 coding. It works.