Recurse into subdirectories when listing the files for a series

This allows for directory layouts such as
Season 1/Series S01E01.mkv
Season 1/Series S01E02.mkv
Season 1/Series S01E03.mkv
...
Season 2/Series S02E01.mkv
Season 2/Series S02E02.mkv
Season 2/Series S02E03.mkv
...
Ignore subtitle files