Refactor last_fm configuration

Move all configuration outside of the LastFm module, so that it's not
polluted with references to the parent music_library application.

lib/music_library/application.ex

@@ -15,7 +15,7 @@ defmodule MusicLibrary.Application do
        repos: Application.fetch_env!(:music_library, :ecto_repos), skip: skip_migrations?()},
       {DNSCluster, query: Application.get_env(:music_library, :dns_cluster_query) || :ignore},
       {Phoenix.PubSub, name: MusicLibrary.PubSub},
-      LastFm.Supervisor,
+      {LastFm.Supervisor, last_fm_config()},
       # Start a worker by calling: MusicLibrary.Worker.start_link(arg)
       # {MusicLibrary.Worker, arg},
       # Start to serve requests, typically the last entry
@@ -40,4 +40,8 @@ defmodule MusicLibrary.Application do
     # By default, sqlite migrations are run when using a release
     System.get_env("RELEASE_NAME") != nil
   end
+
+  defp last_fm_config do
+    Application.get_env(:music_library, LastFm)
+  end
 end