Use Repo pattern for configuration

[danielberkompas]

Some applications might want to talk to multiple separate Elasticsearch stores. The current configuration strategy assumes that you only have one Elasticsearch endpoint.

We should take a page from Ecto's book and allow you to configure multiple Elasticsearch "Repos", something like this:

defmodule MyApp.Elasticsearch do
  use Elasticsearch, otp_app: :my_app
end

You'd then configure MyApp.Elasticsearch like we do now. This would allow a single app (or separate apps within Umbrella apps) to talk to multiple endpoints.

[danielberkompas]

This is addressed in v0.2.0.