Enable selection of Chinese encoding variants

`Big5`, `Big5HKSCS`,  `GB18030` are still commonly seen in the wild, especially when dealing with government- and personal-owned websites developed on Windows machines. This patch enables these encodings for selection.

Encoding names cited from [ashtuchkin/iconv-lite](https://github.com/ashtuchkin/iconv-lite/).
Just curious, should this be `big5` instead? Not 100% sure the difference between `cp950` and `big5` but looks like iconv-lite supports both which is what Atom uses for encoding/decoding.