Update linter to error on compound ampersand selectors

[marlonmarcello]

Let's see if we can find or create a Stylelint rule which prevents things like this. The issue in this example being that you cannot do a project-wide search for .card__description.

.card {
  &__description {
    // some code
  }
}

[andrewrubin]

#84 does half of the work here, but I think we may need to dig into some complex regex to achieve the ampersand request. I looked around but didn't find an existing Stylelint rule for it :(

[andrewrubin]

Updated the description here since we've already solved half of what the original issue was asking for 😄