Inform the developer that they've missed `draggable="true"` on the drag handle

I've just spent more time than I'd care to admit debugging an issue which I initially thought was caused by attempting to use `ember-drag-sort` within an `ember-paper` dialog, as I'd been happily using it outside the dialog, but it didn't want to work at all within it. It turned out that I'd missed `draggable="true"` on my drag handle - but there was no indication that this was the case (other than nothing being draggable).

It would be ideal if either:
- The drag handle didn't require `draggable="true"` (though I'm not sure how feasible that would be);
- The drag handle automatically had `draggable="true"` set on it if missing;
- `ember-drag-sort` raised some sort of warning (at least in dev) if a handle selector is provided but the handle doesn't have `draggable="true"`; or
- The `handle` selector was ignored if the handle didn't have `draggable="true"`, perhaps by adding an attribute selector to the `handle` selector in the `drag-sort-item`'s `$handle` computed property and using `$handle` instead of `handle` in the `draggable` computed property.

If it'll help, I can look at submitting a PR - just let me know what the preferred approach is.