Add callbacks for when the Done button is tapped

[jsahoo]

This PR adds a delegate method and completion handler that are called when the Done button is tapped. This is useful for displaying an activity indicator or HUD while the newly cropped image is being created (as this can sometimes take a second or two with large images), or for tracking button tap analytics.

Example Usage (Delegate)

func cropViewControllerDidTapDone(_ cropViewController: CropViewController) {
    // show activity indicator
}

func cropViewController(_ cropViewController: CropViewController, didCropToImage image: UIImage, withRect cropRect: CGRect, angle: Int) {
    // hide activity indicator
}

Example Usage (Completion Handler)

cropViewController.onDidTapDone = {
    // show activity indicator
}

cropViewController.onDidCropToRect = { image, cropRect, angle in
    // hide activity indicator
}

[TimOliver]

LGTM! Thanks so much for that Jonathan!