Support for CLI Saving/Conversion

[Zedjones]

Currently, it seems like SKIV is the only tool that properly preserves all HDR metadata when converting from a JXR to PNG.

However, there's no way to do so on the CLI (as best I can tell), meaning that it's rather arduous to convert batches of images using the tool and impossible to automate said conversion.

Would it be possible to add the ability to not only open files via the CLI, but also save/convert them? And if it's already possible, could the documentation be updated to reflect this?

[Zedjones]

Well, seems like jxr_to_png does exist and functions fine, actually, but I still think this would be a nice feature to have since SKIV supports more formats than just PNG.