Add Korean support

[KangWH]

This commit adds Korean styles. There are three variants: plain, corners, and angles.

• plain uses quotation marks, which is the same as American English. This fits with modern Korean.
• corners uses corner brackets, which is the same as Japanese. This fits with classic (old) Korean and vertiacal typesetting.
• angles uses angle brackets. Some special typesetters might use this style variant. It also seems that North Korea uses these symbols.

Sample usage:

\documentclass{article}

\usepackage[hangul]{kotex}
\usepackage{csquotes}

\begin{document}

\setquotestyle[quotes]{korean}
\enquote{\enquote{철수}야, \enquote{밥} 먹자!} 하고 철수네 엄마가 불렀다.

\setquotestyle[corners]{korean}
\enquote{\enquote{철수}야, \enquote{밥} 먹자!} 하고 철수네 엄마가 불렀다.

\setquotestyle[angles]{korean}
\enquote{\enquote{철수}야, \enquote{밥} 먹자!} 하고 철수네 엄마가 불렀다.

\end{document}