rebar3 Support

[oubiwann]

I'd like to add Flavors to the OLP project as an included library ... but in order to do so, I'll need to update Flavors with a rebar.config file that rebar3/relx will use when creating an OLP release that includes flavors.

Would you be okay with me adding a rebar.config file in a PR?

[rvirding]

Lay it on me. I will have to cleanup a little bit. Also we would need to specify which directories would be in the release.