Add support for East Asian Width.

This is a real quick hack to replace wcwidth(3) with a version that
takes East Asian (CJK) ambiguous characters into account.

A new option -w is added to mosh(1), which is enabled by default if
run under a CJK locale.
Merge branch 'east-asian-width' of github.com:ykasap/mosh into east-asian-width