Add NestingDepth linter

Fixes issue #246. Please review. This is my first attempt to add NestingDepth. Feel free to test and suggest changes.
Sorry, my comment was wrong. This condition is necessary to reset the `@depth` to 0 if the parent node the node is a RootNode, meaning, if it's a first level rule.

```scss
.one {
}
// reset to 0
.two {
}
```

I changed the comment to make it more clear. Let me know if this approach makes sense.