fix to when valueStepper is in a tableViewCell, autorepeat = true, and press on +/- after scrolling

[skibadawid]

fix to #13 (a noob here so do I need to create an issue first and then do a PR?)

this part of code fixes issue where incrementing/decrementing is in infinite loop and when pressed + or - stops the it, attached a gif. also removes redundant continuousTimer?.invalidate() calls that can be replaced with continuousTimer = nil

private var continuousTimer: Timer? {
    didSet {
        if let timer = oldValue {
            timer.invalidate()
        }
    }
}

Adding an event action .touchCancel fixed the issue with infinite increment/decrement. choose to add it to decrease()/increate selector so its responsive to user interaction, but could be attached to the stopContinuous of increase and decrease button. [around line 140]

decreaseButton.addTarget(self, action: #selector(decrease(_:)), for: [.touchUpInside, .touchCancel])
increaseButton.addTarget(self, action: #selector(increase(_:)), for: [.touchUpInside, .touchCancel])

[BalestraPatrick]

This is great @skibadawid! If you find a bug and create a PR for it, no need to create a issue but this time it was nice because you added the GIF so I could see the bug visually. I never tested ValueStepper in a UITableView so I am happy you are fixing these edge cases. Thanks a lot!