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.
Thanks, but you extended the summary line, and as you can see [here](https://github.com/rebar/rebar/pull/317/commits), [here](https://github.com/UglyTroLL/rebar/commit/327933af7eea5616474139bceaaccefe159eb918), and [here](https://github.com/UglyTroLL/rebar/commit/327933af7eea5616474139bceaaccefe159eb918.patch), the summary is too long.

While fixing that, please also squash the overlong line fixup commit into the original commit.