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/).
According to `iconv-lite`'s [source](https://github.com/ashtuchkin/iconv-lite/blob/master/encodings/dbcs-data.js#L127), seems that `big5` is mapped to `big5hkscs` whilst `cp950` stands for older (Windows XP and earlier) encoding standard. They aren't fully compatible and since Chrome also offers two distinct options for Big5 / Big5-HKSCS, I reckon we should probably enlist both. :P