Guss v3

New syntax for get-line-height and get-font-size:

```scss
.get-font-size {
    font-size: get-font-size(headline, 6); // used to be ($fs-headlines, 6)
}
.get-line-height {
    line-height: get-line-height(header, 3); // used to be ($fs-headers, 3)
}
```

The rest is basically the same, with only one change: [SassDoc](https://github.com/SassDoc/sassdoc) inline documentation. See [this demo of Guss SassDoc-ified](http://www.kaelig.fr/gu/guss-sassdoc/).

Fully fixes #4704

![ ](http://gifs.joelglovier.com/oprah/tom-cruise-zap-oprah.gif)