[Feature Request] Monochrome output with HEX color description

[klei22]

Would be cool to have a flag for monochrome output in the command line utility, maybe something like:

image2ascii --monochrome -f image.png

or

image2ascii -m -f image.png

Even better if one could specify the color with hex (for example green):

image2ascii -f image.png -h 00FF00