Protobuffs compiler take more opts other than just erl_opts

Basically, this issue was brought up by https://github.com/rebar/rebar/pull/150 but it failed to get merged back. I did some improvements on that pull request and Im trying to bringing it up again since I believe it is pretty important for some ppl like me.
This change is unrelated to the patch's main goal, and it's fine to include because it fits in with what abnf_compiler and neotoma_compiler print. However, it's bad style ([Structuring your commits](https://github.com/rebar/rebar/blob/master/CONTRIBUTING.md#structuring-your-commits)) to not use a separate commit for this. In this case, adding an extra note (like `While at it, made the log message more specific.`) in the original commit message is also fine. Does that make sense?