Add formatting string token for subfolder names

[HDoujinDownloader]

Some sites require the recursive traversal of albums, which generates subfolders beyond those created by the chapter-based organization (e.g. 8muses). While the chapter name can be accessed through the %CHAPTER% token, there is no way to access the deepest subfolder name (i.e. the album name) if you wanted to include it as part of your image filenames (discussion here).

Perhaps a new token such as %FOLDER% could be added for this purpose.