Adding support for 256-color and binary images

[sjorskoot]

In #33 (comment), @kovidgoyal mentions the f=1 and f=8 format specifiers for binary images and 8-bit images respectively. I think that's a fantastic idea.

Rationale
As the image graphics protocol is getting more traction with multiple terminals adding support, we can expect more and more CLI developers implementing graphics into their applications. I'm not talking about images, but custom graphics such as interactive charts. In order to make these graphics feel native to the application, the colors should match the text colors used by the terminal. Currently is possible by querying the color scheme, but that is not viable in every scenario.

f=1
Uses 1 bit per pixel: 1 is the current foreground color and 0 is the current background color.

f=8
Uses 8 bits per pixel, or the standard 256 ANSI indices. The first 16 can be used to make colorscheme-aware graphics.

I believe adding these formats will really help making graphics first-class citizens in modern CLI applications. I'm willing to try my hand at an implementation if this is something that we want.

(bonus points for whoever recognizes this)

[kovidgoyal]

Say a bit more about use cases, what would you use these graphics for that you cant already do with the various unicode block characters?