new default: no public plaintext listener

[slingamn]

@DanielOaks I won't merge this without your approval.

I feel like public plaintext is a bad default for 2020, and goes against the ideas in ircv3/ircv3-ideas#45. But if you think this is an unfriendly default, or insufficiently batteries-included, I get it.

@csmith I'm not 100% clear on how this would interact with the Docker configs; would 6667 still be exposed despite this change, because of EXPOSE and -p?

[csmith]

Yeah, if you're running in docker per the README in dist/docker then it will still expose 6667.

But docker's not doing anything clever - if the ora binary isn't listening on 6667 then connections will fail in basically the same way they'd fail if it was running on bare-metal.

(If this lands we should update the docker bits though, as it'll be weird and confusing to expose a port we're not listening on!)

[DanielOaks]

Yep, let's do this