[Feature Request] Monochrome output with HEX color description

Would be cool to have a flag for monochrome output in the command line utility, maybe something like:

```sh
image2ascii --monochrome -f image.png
```

or

```sh
image2ascii -m -f image.png
```

Even better if one could specify the color with hex (for example green):

```sh
image2ascii -f image.png -h 00FF00
```
