Added constant NG for HANGUL LETTER IEUNG

It's handy when IEUNG used as JONGSEONG.
Because IEUNG sound '-NG' at JONGSEONG place.
SplitMultiElement: fixed to conver given rune to compat jamo
jamo.go: added IsLead, IsMedial, IsTail
encoding/cp949: Add support for Korean native encoding, cp949
encoding/cp949: Use pre-sorted lookup table
encoding/cp949: add license header and doc