Support URLs from Alpha Coders Wallpaper Abyss

[wleoncio]

Version of Variety you are using
0.8.13

Is your feature request related to a problem? Please describe.
I would like to add a URL from https://wall.alphacoders.com/, but there's no proper place to do so. As expected, pasting it to options expecting URLs from other sites doesn't work.

Describe the solution you'd like
An "Alpha Coders Wallpaper Abyss" option on the "General > Add..." button that takes downloads images from URLs such as https://alphacoders.com/pixel-art-wallpapers

Additional context
Here's where I imagine the new option