Generate styles with sprite dimensions

It will be usefull to generate css, less or any other stylesheet for sprites, for e.g.:
```
icon--arrow {
width: 32px;
height: 64px;
}
```
where width and height based on svg size attributes or viewBox.

The problem for me is that when inserting sprites via the `` tag, it has default SVG dimensions (300*150) no matter what size the icon is, so I need to manually write the dimensions for each icon in my stylesheets.

If this feature is welcomed, I may try to do some PR.