[Feature request] dynamic image providers

[Be-ing]

A feature I love in KDE is dynamic image providers for the desktop wallpaper. This is distinct from #69 in that these image providers would show static images. In KDE, I use an image provider that shows NASA's Astronomy Picture of the Day. The code for that image provider simply downloads that webpage once a day and gets the image URL from the page's HTML.

Here is a screenshot of KDE's settings for this: