NFT could be improved by adding support for transparency

[Marco4413]

I've been thinking for the last few days that it would be cool if you could change the background/foreground color for pixels on nft images, just like nfp does you could set the color of the background/foreground to a space character to make it transparent.

I was also thinking that if transparency would be added the color of the foreground (if transparent) could be the same as the background of the pixel underneath (in case there was a screen buffer that would support it else the color of the terminal's background).