Added --gen-name-strings command line option.

To support the use case described in issue google/flatbuffers#3826, a new command line option --gen-name-strings
has been added, which will cause a static GetFullyQualifiedName function to be added
to the C++ output for tables/structs.

include/flatbuffers/idl.h

@@ -321,6 +321,7 @@ struct IDLOptions {
   bool proto_mode;
   bool generate_all;
   bool skip_unexpected_fields_in_json;
+  bool generate_name_strings;
 
   // Possible options for the more general generator below.
   enum Language { kJava, kCSharp, kGo, kMAX };
@@ -339,6 +340,7 @@ struct IDLOptions {
       proto_mode(false),
       generate_all(false),
       skip_unexpected_fields_in_json(false),
+      generate_name_strings(false),
       lang(IDLOptions::kJava) {}
 };