添加了动画过渡效果

[zhangruize]

Add animation between every setProgress.

[daimajia]

Hi @zhangruize , It's really a marvelous feature for this tiny library!

There are some suggestions on the code part, hope you can do some update.

• add a comment on the ANIMATION_DURATION varaible, and it's better to allow developers to modify the animation_duration
• allow developers to customize the Interpolator
• allow developers to enable or disable this feature.

Really thanks for your contribution on this project. 🌹 Looking forward to your update on this pull request.