Support for :mapfields_as_maps

Hey there!

Recently I've opened an issue on [`grpc-elixir`](https://github.com/tony612/grpc-elixir/issues/9) trying to figure out a way to parse proto's maps as Elixir's maps rather than a list of tuples (default).

Taking a look at `exprotobuf` and `gpb`, I saw that the second [accepts an option](https://github.com/tomas-abrahamsson/gpb/blob/b2ed4c87f3f4629a1eb4bb24abc29b4263bc5ffd/src/gpb_compile.erl#L248) for such kind of map parsing. Then I tried to pass both options `[:mapfields_as_maps]` and `[{:maps, :mapfields_as_maps}]` to the [parse functions](https://github.com/bitwalker/exprotobuf/blob/master/lib/exprotobuf/parser.ex) but it didn't work as expected.

I couldn't find any test/spec covering this case so I'm just wondering how should I proceed to make use of this option and be able to use Elixir maps on my map fields.