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/last_fm/supervisor.ex

@@ -1,28 +1,19 @@
 defmodule LastFm.Supervisor do
   use Supervisor
 
-  def start_link(init_arg) do
-    Supervisor.start_link(__MODULE__, init_arg, name: __MODULE__)
+  def start_link(config) do
+    Supervisor.start_link(__MODULE__, config, name: __MODULE__)
   end
 
   @impl true
-  def init(_opts) do
+  def init(config) do
     :ok = LastFm.Feed.create_table!()
 
     children = [
       {Phoenix.PubSub, name: LastFm.PubSub},
-      {LastFm.Refresh, %{api: api(), user: "cloud8421", api_key: api_key()}}
+      {LastFm.Refresh, config}
     ]
 
     Supervisor.init(children, strategy: :one_for_one)
   end
-
-  defp api_key do
-    Application.get_env(:music_library, LastFm, [])
-    |> Keyword.get(:api_key)
-  end
-
-  defp api do
-    Application.get_env(:music_library, :last_fm, LastFm.APIImpl)
-  end
 end