Allow mixins outputting selectors to be used at the stylesheet root

[JohnAlbin]

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);

[chriseppstein]

Script parent didn't get into Sass 3.3. Instead, I've added a custom function at-stylesheet-root() which returns true when & used to return null. Can you rework this patch to use that instead?

See: 5fb0b88

[chriseppstein]

@JohnAlbin Also, this needs tests :)

[chriseppstein]

@JohnAlbin Sass 3.4 is being released today. want to dust this off for 1.1?