Add unicode block character support about img2xterm

[rossy]

That's a good idea, but it also adds other complexities. For example, the half blocks ▀▄▉ are almost square with most terminal fonts, but I think the quarter blocks are less square, so it wouldn't look as good with pixel art. The colour thing is a killer too, because it's pretty easy to pick optimal colours when you can set each colour independently, but it would need more complicated code if it had to find an optimal 2-colour palette for each block of four pixels.

Since this is a big change and this project is somewhat unmaintained at the moment, I can't promise anything, but I'll keep it in mind.

[klopsi]

ok if you don't mind, i'll link to my unicode block character exporter and related tools
https://github.com/klopsi/unicolexport

cheers!