Rework `process_image()` to Generically Accept Keyword Arguments

[jrg94]

Right now, process_image() is blowing up with features. We should rewrite this method to generically accept keyword arguments, so we can simplify the contract going forward.

[jrg94]

To make this even cleaner, we should integrate this change at the same time as #51. That way, we can pass our argparse object over almost directly: https://stackoverflow.com/questions/15206010/how-to-pass-on-argparse-argument-to-function-as-kwargs

[jrg94]

To be clear, this change was made universally. The kwargs generated by parse input are used by both process_images and save_copies.