Add directory hierarchy in css classname

With an Directory structure like this:
```
\hierarchical
    bleu.png
    \a
        green.png
        \b
            red.png
```

CSS class name will be :
- .sprite-hierarchical-a-b-red
- .sprite-hierarchical-a-green
- .sprite-hierarchical-blue

This allow me to have 2 images with the same name in different directory.