Extract image to ascii as a lib

I think it would be nice.
If you're interested here is my take on it https://github.com/sigmaSd/rascii/blob/master/src/lib.rs and an example of usage with corssterm https://github.com/sigmaSd/rascii/blob/master/src/main.rs

The API exposes `image_to_ascii` and an optional default `print_ascii` function (user can write his own)