Implement "Nesting Depth" lint rule

[srawlins]

Need to implement a lint rule that reports style rules that are too deep, in terms of Sass nesting (not selector specificity):

p.foo {
  .bar {
    .error {
      .severe { // LINT
        color: red;
      }
    }
  }
}

This is similar to scss-lint's NestingDepth.