I don't think this code is particularly clear - calling a mixin within a mixin in order to alias it? Why not just `image-replace = ir`?

Also, for more understandable code, should it not be the other way around...?: 

```
image-replace(path, dimensions...)
  font: 0/0 a
  text-shadow: none
  color: transparent
  bg(path) if path
  if length(dimensions) > 1
    width: unit(dimensions[0], px)
    height: unit(dimensions[1], px)
  else
    warn("Make sure you also pass the image dimensions. Example: image-replace: '/img/whatever.jpg' 200 400")

ir = image-replace
```