Delay the presentation of the skeletons

[Juanpe]

Summary

Fixes #379

If using SkeletonView for views that are mostly updated quickly, the SkeletonView transition looks like a short flashing of the view.

The solution was to create two methods to delay the skeleton transition. So, now you can delay the presentation of the skeleton if the views update quickly.

func showSkeleton(usingColor: UIColor,
                  animated: Bool,
                  delay: TimeInterval,
                  transition: SkeletonTransitionStyle)

func showGradientSkeleton(usingGradient: SkeletonGradient,
                          animated: Bool,
                          delay: TimeInterval,
                          transition: SkeletonTransitionStyle)

Requirements

• [x] I've read and understood the Contributing guidelines and have done my best effort to follow them.
• [x] I've read and agree to the Code of Conduct.

[github-actions[bot]]

	Warnings
:warning:	Consider **also** updating the README for other languages.

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