Add `.multipleLine` UI Hint type to `RSDTableStepViewController`

[syoung-smallwisdom]

1. Add a new UITableViewCell subclass that uses a UITextView to enter text and add to the list:

    open func registerReuseIdentifierIfNeeded(_ reuseIdentifier: String)  { 
        ...

2. Add the supported type to the list:

    /// The UI hints that are supported by this view controller.
    open class var supportedUIHints: Set<RSDFormUIHint> {
        return [.list, .textfield, .picker, .checkbox, .radioButton, .modalButton]
    }

3. Handle any custom configuration:

    open func configure(cell: UITableViewCell, in tableView: UITableView, at indexPath: IndexPath) {
        ...