Added --defaults-json to output fields at default value.

Normal behavior is to not output fields that happen to have
the default value, since those will be reproduced anyway
when turned into a FlatBuffer binary. This however can be problematic
when using JSON to interop with other system since they might not
know this default value. This flatc option (and also flag
to GenerateText) will force those fields to be output anyway.

Tested: on Linux.

src/flatc.cpp

@@ -87,6 +87,8 @@ static void Error(const std::string &err, bool usage, bool show_exe_name) {
       "  -M              Print make rules for generated files.\n"
       "  --strict-json   Strict JSON: field names must be / will be quoted,\n"
       "                  no trailing commas in tables/vectors.\n"
+      "  --defaults-json Output fields whose value is the default when\n"
+      "                  writing JSON\n"
       "  --no-prefix     Don\'t prefix enum values with the enum type in C++.\n"
       "  --gen-includes  Generate include statements for included schemas the\n"
       "                  generated file depends on (C++).\n"
@@ -130,6 +132,8 @@ int main(int argc, const char *argv[]) {
         include_directories.push_back(argv[argi]);
       } else if(arg == "--strict-json") {
         opts.strict_json = true;
+      } else if(arg == "--defaults-json") {
+        opts.output_default_scalars_in_json = true;
       } else if(arg == "--no-prefix") {
         opts.prefixed_enums = false;
       } else if(arg == "--gen-mutable") {