fix if a mixin/func was nested in @media

[asimpson]

@robtarr :eyes: on this please? Basically I moved the walkAtRules "up" a level because the un-nesting plugin unwraps media rules so that the SCSS looks like:

@media (min-width: 40em) {
  @include brand(foo) {
    div span span p {
      color: red;
    }
  }
}

walkRules wasn't cutting it, since @ is an atrule, not a normal rule.

I also added this check into the regular brand function parsing.

[robtarr]