Add ability to convert any io.Reader (or maybe any image.Image) to aic_package

[different55]

Having aic_package.Convert handle opening local files or fetching remote files seems like it should be out of scope for a function that's just named Convert. Having it (or another function) handle a generic io.Reader or image.Image would be hugely useful. Right now it's not really possible to use aic_package to ascii-fy an embedded or generated image, for example.

[TheZoraiz]

Yea, the current library support is just cli usage exposed as a package. It's definitely not as refined as it should be. I'll probably create another function that takes image.Image when I implement this.