Support recursive wallpaper directories

[Nathanwoodburn]

G'day,
At the moment I can't store my wallpapers in recursive directories. This would make it easier to sort images while still having a random everything option.

$ tree
.
├── Vietnam
│   ├── HoiAn01.jpg
│   └── HoiAn02.jpg
├── General
│   ├── nord-street.png
│   ├── nord_dark_city.png
│   ├── nord_purple_waves.png
│   └── nord_car_live.gif
└── Thailand
    ├── Sukhothai_03.jpg
    ├── Sukhothai_01.jpg
    ├── Sukhothai_02.jpg
    ├── Sukhothai_05.jpg
    └── Sukhothai_04.jpg
$ WallRizz -d .

  No wallpaper found in ./:
  Make sure the supported image file exists in the directory.
$ WallRizz -d Thailand
 ◉ Caching images...