Support sixel (and other native image displaying protocols)

[matrach]

This repo is getting more attention that I initially anticipated. Since people actually use the tool, it would be nice to support native image rendering in some terminals (most notably through sixel). Reference of modern terminals capabilities: https://github.com/dankamongmen/notcurses/blob/master/TERMINALS.md

Ref: https://github.com/saitoha/libsixel Pure Python: https://pypi.org/project/PySixel/ https://github.com/dankamongmen/notcurses