feature: save multiple outputs

[dribnet]

Currently the program runs and saves one output - the image itself. This feature request is to broaden the idea of an output to potentially include:

• Drawer internal state: For VQGAN this would be the z vector. For PixelDraw this would be colors for each square, etc. This would allow restarting from a previous run, etc.
• Alternate outputs: For ClipDRAW and PixelDraw this would be SVG in addition to the image
• Log file: it would be nice to have a log that included the loss curve, etc. for analysis or debugging
• Others?

These need not be implemented all at once, but they are included here just to showcase that the scope of the refactor should be broader than just tacking on a single feature, but more broadening the scope of what "--output" is meant to mean. Most likely output becomes a directory of files with standard names.

Footnote: I have also wanted to port in my templating code for auto-generating output file names based on date, random seed, etc. and I think it would make sense to do that as part of this change to the semantics of output.