Introduce `js3-label-indent-offset`.

This is a partial solution to #70 and #71, allowing the styles like this:
```javascript
if (cond) {
    switch (state) {
      case 1:
        doA();
        break;
      case 2:
        doB();
        break;
      default:
        return;
    }
}
```
and this:
```javascript
if (cond) {
    switch (state) {
    case 1:
        doA();
        break;
    case 2:
        doB();
        break;
    default:
        return;
    }
}
```

Some additional work will be needed to support the following style.  
```javascript
if (cond) {
    switch (state) {
        case 1:
            doA();
            break;
        case 2:
            doB();
            break;
        default:
            return;
    }
}
```