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).

include/yaml-cpp/emittermanip.h

@@ -19,6 +19,7 @@ enum EMITTER_MANIP {
   // output character set
   EmitNonAscii,
   EscapeNonAscii,
+  EscapeAsJson,
 
   // string manipulators
   // Auto, // duplicate