Allow ignoring @mixin

[raycohen]

Started using this in a project with sass files, and it flags the h1 inside the mixin as unscoped:

@mixin example {
  h1 { color: red; }
}

Would you accept either a PR to explicitly skip @mixin the way @keyframes is currently being skipped, or a PR that adds options.ignoredAtRules to allow the consuming project to configure arbitrary additional at-rules to skip?

[raycohen]

Bumping this - let me know if you'd accept a PR here!