Add support for JSON-compatible string escapes (#485)

For completeness I've implemented escaping for characters outside the
basic multilingual plane, but it doesn't get used (as there's no
EscapeAsAsciiJson emitter option implemented).

src/emitterstate.cpp

@@ -231,6 +231,7 @@ bool EmitterState::SetOutputCharset(EMITTER_MANIP value,
   switch (value) {
     case EmitNonAscii:
     case EscapeNonAscii:
+    case EscapeAsJson:
       _Set(m_charset, value, scope);
       return true;
     default: