Model: Day, month and year for custom filename (bug 1768)

Users can now use the following fields to customize the
sync filename when using MP3 player device sync:

  {episode.pubdate_year} - Year of publication (without century)
  {episode.pubdate_month} - Month of publication (01-12)
  {episode.pubdate_day} - Day of publication (01-31)

In addition to these fields, {episode.sortdate} is still
provided and gives YYYY-MM-DD as result (just like before).