Support Unsplash collections

Hello,
First of all, thanks for this amazing tool.

I wonder if it's possible to support Unsplash collections since the wallpapers curated there are prettier and of a better quality.
I could see an API available for it: https://unsplash.com/documentation#get-a-collections-photos, I haven't looked through the code, but I guess it would be just a matter of an addition to the configuration file.

Cheers