add JSON_C_TO_STRING_COLOR option

This option enable color in json_object_to_json_string_ext.
I've try to made something similar to jq output,
but I've color true/false and null in purple,
as it's what is common color scheme used in programing language in emacs.

also add a '-c' option into json_parser to test it.

note: that I could have done a color() function similar to
what is done with indent(), but as the code is pretty small
I've keep it as it. so if you want  me to use a subfunction
tell me and I'll do it.

Signed-off-by: Matthias Gatto <matthias.gatto@protonmail.com>

json_object.h

@@ -74,6 +74,15 @@ extern "C" {
  */
 #define JSON_C_TO_STRING_NOSLASHESCAPE (1 << 4)
 
+/**
+ * A flag for the json_object_to_json_string_ext() and
+ * json_object_to_file_ext() functions which causes
+ * the output to be formatted.
+ *
+ * Use color for printing json.
+ */
+#define JSON_C_TO_STRING_COLOR (1 << 5)
+
 /**
  * A flag for the json_object_object_add_ex function which
  * causes the value to be added without a check if it already exists.