Implement extend for wrapped selectors

[mgreter]

Fixes #1797

A more complete spec test which has some logic I don't yet understand:

%not {
  color: red;
}
.not {
  @extend %not;
}
div:has(%not) {
  color: black;
}

bar {
  span:not(%not) {
    color: black;
  }
  span:not(&.foo) {
    color: black;
  }
  span:not(&%not) {
    color: black;
  }
}

Ruby sass result:

.not {
  color: red; }

div:has(.not) {
  color: black; }

bar span:not(.not) {
  color: black; }
span:not(bar.foo) {
  color: black; }
span:not(bar.not) {
  color: black; }

Libsass output with this PR:

.not {
  color: red; }

div:has(.not) {
  color: black; }

bar span:not(.not) {
  color: black; }

bar span:not(bar.foo) {
  color: black; }

bar span:not(bar.not) {
  color: black; }

It seems to follow the logic of this sample:

foo {
  span:not & {
    color: black;
  }
}

Which yields in both:

span:not bar {
  color: black; }

[xzyfer]

Spec added and activated in sass/sass-spec#690