Use EUC-KR only for internal conversion
Extend the range of BaseHangul

See
https://github.com/koreapyj/basehangul/commit/8a5026dd62ee2e4771dfd6831820b568fd4d9e32
and http://api.dcmys.jp/basehangul/.
Move util methods to Utils module
Implement encode and decode of BaseHangul