[generator] Move options parsing into dedicated class

[Yannic]

*** Note: There is no behavior change from this patch. ***

[stanley-cheung]

Thanks for the contribution. Yes this refactoring looks good. There's some test failure, PTAL, thanks

g++ -std=c++11 -I/usr/local/include -pthread  -c -o grpc_generator.o grpc_generator.cc
grpc_generator.cc: In member function 'virtual bool grpc::web::{anonymous}::GrpcCodeGenerator::Generate(const google::protobuf::FileDescriptor*, const string&, google::protobuf::compiler::GeneratorContext*, std::__cxx11::string*) const':
grpc_generator.cc:1534:41: error: 'class grpc::web::{anonymous}::GeneratorOptions' has no member named 'file_name'; did you mean 'file_name_'?
         context->Open(generator_options.file_name(file->name())));
                                         ^~~~~~~~~
make[1]: *** [grpc_generator.o] Error 1
<builtin>: recipe for target 'grpc_generator.o' failed
make[1]: Leaving directory '/github/grpc-web/javascript/net/grpc/web'
Makefile:101: recipe for target 'install-plugin' failed

[Yannic]

Looks like I didn't add everything to the commit. Sorry about that!
ptal, thanks!

[stanley-cheung]

Nit: our internal c++ linter doesn't like this ; after a }. Please remove it. Thanks!

[Yannic]

Thanks, done!