Warn users about elements not displayed (e.g. images)

[giulianopz]

Try to display images with SIXEL graphics: https://github.com/mattn/go-sixel/tree/master

References:

• https://nick-black.com/dankwiki/index.php/Sixel
• https://nick-black.com/dankwiki/index.php/Theory_and_Practice_of_Sprixels
• https://vt100.net/docs/vt3xx-gp/chapter14.html
• https://vt100.net/shuford/terminal/all_about_sixels.txt
• https://www.arewesixelyet.com/