Add config::Etcd to merge configuration from etcd

[dtolnay]

[mehcode]

This is done on the etcd branch if someone would like to try it out and let me know how it works for them. I'm going to let it sit there for a bit and tackle cleaning up the code before merging it into master.

[matthiasbeyer]

I started rebasing the work of @mehcode from the "etcd" branch onto master, but it seems that the etcd crate uses an old version of the futures and tokio ecosystem, thus I would not further invest time in this for the time being, at least not with this crate.

@dtolnay if you're interested in getting this feature implemented, I'd be more than happy contributing feedback, review and possibly code - I never worked with etcd, but I guess that's a feature we should implement in this crate, if someone wants to step up!

[matthiasbeyer]

Related to #148