Support sixel images

[jficz]

Current image support is rudimentary and quickly becomes useless if font is enlarged for big screen presentations.

Sixel graphics support would solve this problem at least partially by significantly increasing the displayed image resolution.

With "normal" font size the image looks "acceptable" but the text is not presentable.

When the font is enlarged for the presentation to be (barely) suitable for big screens, the image is practically useless:

Using sixel graphics rendering, the logo is very nicely legible even at maximum font size supported by my terminal: (this is a screenshot of broot's preview feature and is an actual screenshot of the terminal window - kitty in this case)