Add property to allow multiple gesture recognizers

To enable other gesture recognisers that might be affected by the UIPanGestureRecognizer instances used for opening and closing a drawer, like the swipe to delete UIPanGestureRecognizer on UITableViewCell instances that have UITableViewCellEditingStyle == UITableViewCellEditingStyleDelete, implement the UIGestureRecognizerDelegate shouldRecognizeSimultaneouslyWithGestureRecognizer method.