Reverting to center of element

Sometimes you might want to have your helper revert to the center of your element instead of the top left. This happens in case your helper is bigger/smaller than your element, in that case it feels unnatural to make it revert to the top left.

I've added a `revertTo` property that accepts one of the following values:

- `original`: Reverts to the top left position of the element
- `center`: Reverts to the center of the element