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
}
``` 
Using `let` statements instead of instance variables matches the conventions used in the rest of our tests.