nodeToSketchGroup - `flatten` option
As discussed in https://github.com/chrisvxd/story2sketch/issues/20

### Before

`nodeToSketchGroup(node);`

```
(group A)
  (group B)
     (group C)
        layer
```

### After
`nodeToSketchGroup(node);`

```
(group C)
   layer
```

❓Or should it be group A? Or maybe some new group that contains names of all A, B & C? 🤔
