[Feature] Ability to listen for `repositioned` event

[patrickberkeley]

When using constraints, it would be handy to be able to listen to the repositioned event that was added and described in this PR.

The main use case I know if is to be able to handle updating the offset dynamically when the position is changed by hitting a constraint (described in this issue).

[lukemelia]

@patrickberkeley Sounds like this would be a suitable addition to ember-tether. I'm happy to offer feedback on API and PR to you or anyone interested in implementing.