Implement shouldTransition(for scrollView: UIScrollView?) delegate method

[brandonscript]

I had a need to be able to govern from the presented view controller whether or not a particular scroll view should respond and interactively dismiss the controller. This was driven from a need to handle a re-orderable table view, and the long-press/drag gesture was being intercepted, and closing the view.

Simple implementation of this delegate, for example in my case, I needed to block the transition if my tableView is editing:

extension ModalViewController: DeckTransitionViewControllerProtocol {
    func shouldTransition(for scrollView: UIScrollView?) -> Bool {
        return !editHistoryTableView.isReordering // returns true if the table view is not editing
    }
}

[HarshilShah]

Hey there, thank you for this PR, and sorry for the late response.

I’m working on an update to the library which makes a whole range of changes, one of which will be the ability to selectively enable/disable dismissal. You can read more about it on #109. Since the implementation is going to be a bit different there and more generic, closer to the way Apple’s done it, I’m going to close this PR.