for those who can not access `raw.githubusercontent.com` normally, they can install this package with the following command
```
PNGQUANT_BIN_BASE_URL=https://npm.taobao.org/mirrors/pngquant-bin/ npm i -S pngquant-bin
```

• Commit edf94ce
• #115: make bin-wrapper read url value from env vars.
• Branch master

littlee