Support rubocop style scss-lint:disable and scss-lint:enable comments

Addresses #150 

This allows for rubocop style scss-lint:disable and scss-lint:enable comments. Examples follow.

I'm not sure if this is a satisfactory minimal solution for everyone, happy to make changes to make it fit in (we really would like this feature).

Applies for the entire file:
```
// scss-lint:disable BorderZero
p {
  border: none; // Does not lint
}
``` 

Disables a few linters:
```
// scss-lint:disable BorderZero, StringQuotes
p {
  border: none; // Does not lint
  content: "hello"; // Does not lint
}
``` 

Applies within the rule it is applied at (and all descendent rules):
```
p {
  // scss-lint:disable BorderZero
  border: none; // Does not lint
}

a {
  border: none; // Lints
}
``` 

Disable and enable again:
```
// scss-lint:disable BorderZero
p {
  border: none; // Does not lint
}
// scss-lint:enable BorderZero

a {
  border: none; // Lints
}
``` 

Support disable/enable all:
```
// scss-lint:disable all
p {
  border: none; // Does not lint
}
// scss-lint:enable all

a {
  border: none; // Lints
}
``` 
The one unfortunate side-effect of this approach is that any linter which defines `visit_comment` will override this implementation, so unless the subclass remembers to call `super` it will inadvertently break this functionality.

At the moment, this only affects the trivial `Comment` linter, but when #161 finally gets implemented who's to say whether this will be taken advantage of more often.

With this in mind, the easiest workaround is to rename this method to something like `parse_control_comment` and call it in the `visit` method above, right before `super`, e.g.

```ruby
if node.is_a?(Sass::Tree::CommentNode)
  parse_control_comment(node)
end

super

...
```