Add Erlang Support

[kazk]

I've started looking into adding Erlang support.

• Erlang/OTP 20.0 (2017-06-21)
• Use rebar3
• Output using eunit_listener

erlang-runner is locked to Erlang/OTP 18.x for Elixir 1.2.4 (#473).
I'm thinking of just making a new erlang-runner and turning the existing one into elixir-runner.

[jhoffner]

I'm thinking of just making a new erlang-runner and turning the existing one into elixir-runner

That's fine if that's what needs to happen. Ideally they would be on the same image just to minimize overhead (container overhead, ram overhead, startup overhead, image management, testing, etc). The list of containers is getting pretty large - but at this point we may just have to accept this as business as usual.