feat: artwork tag option

[jpjagt]

I added basic support for an --artwork option that you can use to specify a path to an image that will be read and added as the artwork audio tag for the output tracks.

I added it for myself and thought, I'll make a PR -- maybe it's appreciated as a general feature. It doesn't add much complexity, just makes sure to read in the image file if it's supplied.

LMK what you think! Thanks for the great tool.