Allow mixins outputting selectors to be used at the stylesheet root

I was converting a CSS ruleset of:

```
::selection {
  background: #ca322c;
  color: white;
}
```
to use Compass, when I saw that the Compass mixin required a `* { }` wrapper. With Sass 3.3's `#{&}` capability, it should be possible to use any mixin that outputs a selector at the root level of the stylesheet.

Currently, doing this at the root of a stylesheet produces an error, but it shouldn't:

```
@include selection(#ca322c, white);
```