CLI <save-screen>: Add --printable option

[atx]

The commit adds the ability to generate black and white screenshots which are ready to be printed. e. g.

[pklaus]

A very useful option, thanks!

[pklaus]

I made a few more changes resulting in

• even more printable screenshots (white is plain white now) and
• smaller filesizes by dropping the alpha channel (plus converting to grayscale in case of --printable).

All those changes are now contained in v0.3.4 (on PyPI).

Thanks again.