introduce the Gnat.Supervisor macro

[mmmries]

This PR is an attempt to create a unified interace as described in #62

It follows a similar pattern to Ecto.Repo by allowing clients of the library to define their own module like this

defmodule MyApp.Gnat do
  use Gnat.Supervisor, otp_app: :my_app
end

Then in their application configurations they can provide configuration like

import Config

config :my_app, MyApp.Gnat,
  connection_settings: [%{host: ~c"localhost", port: 4222, ...}],
  consumers: [
    %{module: MyApp.Server, subscription_topics: [%{topic: "rpc.MyApp.search", queue_group: "rpc.MyApp.search"}]
  ]

Then they can add a single entry to their application supervision tree

children = [MyApp.Gnat]

And we'll start the ConnectionSupervisor, ConsumerSupervisor and other supervisors automatically for them. The user can also call functions like MyApp.Gnat.pub/2 where they don't have to specify the name or pid of the connection.