Unicode support in charts
*Created originally on Bitbucket by [mcfoton (Mikhail Nazarov)](https://bitbucket.org/%7B6a988ae3-5c9c-4e9f-836c-ec7780a82be0%7D/)*
Hey guys. Thanks for awesome work!
I've tried create a chart and got
```
#!
'ascii' codec can't encode characters in position 0-9: ordinal not in range(128)
Exception Location: .../openpyxl/utils/__init__.py in quote_sheetname, line 182
```
The string that could not be encoded/decoded was: **Количество пост**
Tried to avoid it but got into same error with reference.py line 72
issue