Add context function to scrobble a release

lib/music_brainz/release.ex

@@ -25,6 +25,10 @@ defmodule MusicBrainz.Release do
     Enum.sum_by(medium.tracks, fn track -> track.length || 0 end)
   end
 
+  def tracks(release) do
+    Enum.flat_map(release.media, fn medium -> medium.tracks end)
+  end
+
   def from_api_response(r) do
     %__MODULE__{
       id: r["id"],