Feature request: support sixel format

[data-man]

Many terminals supports sixel only.

Proposals:

• migrate to ImageMagick (which supports sixels out-from-the-box) or
• use libsixel

[hzeller]

Added in https://github.com/hzeller/timg/commit/572c76339970bca41c273b65cce57b1b379c521d

Still needs some final tests, but looks pretty promising. Can be chosen with -psixel

[hzeller]

Finished tests. Seems to work with the various terminals I tested (including needed worarounds)

• xterm -ti vt340
• mlterm
• konsole (though by default timg will use -p iterm there)
• xfce4-terminal (compiled with latest vte that allows for sixels)
• wezterm
• alacritty-sixel

Will be in a release shortly.