Allow sub-classing of `Row`

If we allow users to sub-class `Row` and let them pass it to [`createRow`](https://github.com/offirgolan/ember-light-table/blob/0edf5d0adace49a63de1eb1cbc52ada0cf4188ed/addon/classes/Table.js#L400-L410) or even better yet let them specify it for the `Table` instance, they could add custom and more sophisticated behavior to a single row and even add ember-concurrency tasks to it.