add option to CLI and pngwriter that allows user to set PNG compression level

- In CLI: the argument is --png_compression <0..9> (-z<0..9>)
- In API, pass `compress_level` to PngWriter.save_image_and_prompt_to_png()

Compression ranges from 0 (no compression) to 9 (maximum compression).
Default value is 6 (as specified by Pillow package).

This addresses an issue first raised in #652.

scripts/invoke.py

@@ -270,6 +270,7 @@ def main_loop(gen, opt, infile):
                             model_hash = gen.model_hash,
                         ),
                         name      = filename,
+                        compress_level = opt.png_compression,
                     )
 
                     # update rfc metadata