Add compiler option to swap out using module

[lizziepaquette]

Adds the parameter using_module so that one can switch out the using macro and take over code generation. 💪

Example:
myproto.proto

syntax = "proto3";

package my.proto.bar;

message Foo{
  string field1 = 1;
}

protoc --elixir_out=./lib --plugin=./protoc-gen-elixir --elixir_opt=using_module=MyMod.MyProtobuf myproto.proto

defmodule My.Proto.Bar.Foo do
  @moduledoc false
  use MyMod.MyProtobuf, syntax: :proto3

  @type t :: %__MODULE__{
          field1: String.t()
        }
  defstruct [:field1]

  field :field1, 1, type: :string
end

[mairbek]

let's not use master, rather a different branch?