Consider @output for mixins

Mixins do not return a value, they output code. what about a `@output` annotation exclusively for mixins?

Now, either we keep things simple:

``` scss
/**
 * @output Description
 */
```

Or we want complicated things where we list all properties that can be output.

``` scss
/**
 * @output font-family, font-weight, color
 */
```

Any thought?
