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/).
Okay, cool, so should this PR be updated then to support both? Or is this good to merge as is?