Added Listen.Skipped attribute

This will help implementing similar functionality for other formats
that make this distinction.

internal/backends/spotifyhistory/models.go

@@ -91,6 +91,7 @@ func (i HistoryItem) AsListen() models.Listen {
 		ListenedAt:       i.Timestamp,
 		PlaybackDuration: time.Duration(i.MillisecondsPlayed) * time.Millisecond,
 		UserName:         i.UserName,
+		Skipped:          i.Skipped,
 	}
 	if trackURL, err := formatSpotifyUri(i.SpotifyTrackUri); err != nil {
 		listen.AdditionalInfo["spotify_id"] = trackURL