(probably very easy) Allow picking background color for PNGs with transparency.

[JulesGM]

Right now, the background color of transparent images is made white automatically. It would be nice if one could input an RGB value as a command-line argument to specify a different color.

Would likely be very straightforward to implement for someone with knowledge of the project.

[JulesGM]

I wanted to mention that this project i very cool by the way!

[aaronliu0130]

@stefanhaustein Good first issue, try it?