Compiled file path

[ajiyoshi-vg]

• I made a rebar3 plugin using grpc_lib.
• It's almost the same as rebar3 gpb plugin but it uses grpc_lib to automatically generate code for grpc.
• gpb has some options to change the file name/directory of the output source code.
  • For example, you can specify {module_name_suffix, "_pb"} or {module_name_prefix, "pb_"}, then get pb_something.erl or other_pb.erl
  • or just {o, "dir"} to specify output directory
• These options of gpb will be respected by this pull request.