Adds the ability to make thumbnails square, both file and embedded. Usage:
```
my_preset:
square_thumbnail: True
```
Will soon enable this by default for all music-based presets with a toggle to disable.
Huge thanks to @Kentaro1043 for crafting the ffmpeg command for this!
Closes https://github.com/jmbannon/ytdl-sub/issues/383
fc2da4d525