Extend DuplicteRoot to MergeableSelector

Basically its doing the same as the existing linter, and not only on the root element but also on every other rule.

There is a new option `force_nesting` that ensures, thinks like this:

**Bad**
```
h1 {
  color: #fff;
}
h1.new {
   color: #000;
}
```

**Good**
```
h1 {
  color: #fff;
  &.new {
    color: #000;
  }
}

I tried it on an existing code base and it worked fine - but I'm sure its not perfect jet, so let me know what you think about it.