Fetch config from remote sources

[Chris Keathley]

Vapor should be able to retrieve configuration from remote sources such as etcd, zookeeper, or vault. There should be a provider behaviour or protocol to implement for adding new providers. We should look at how this is managed in distillery to see if we can conform to the same behaviour: https://github.com/bitwalker/distillery/blob/master/lib/mix/lib/releases/config/providers/elixir.ex