rebar3 protobuffs provider

Provider to compile protobuffers files using

Place your `.proto` files in `src` directory and they will be automatically


Add the plugin to your rebar config:

    {plugins, [rebar3_protobuffs]}.

The compile function is under the protobuffs namespace. To automatically compile `.proto` files before
the Erlang compiler add the pre_hook to rebar.config:

{provider_hooks, [
                 {pre, [{compile, {protobuffs, compile}}]}