New Feature: Grabbing Clear Logos

[KellyNewman87]

With clear logos as the new thing in Plex to go along with posters and backgrounds, it would be great if grab-all-posters.py was able to retrieve these logos from Plex as well.

Plex has said they will be adding to their UI the ability to update clear logos, so it seems to make sense to have a tool that grabs these changes to the clear logos just like grab-all-posters.py does for the changes made in Plex UI to posters and backgrounds:

https://forums.plex.tv/t/only-localized-logos-are-showing-up/919365/24

However, even if Plex doesn't add this to the UI, it would be very helpful to grab all the current logos to help populate Kometa's assets folder.