[Enhancement] Add a way to animate layer

[ghost]

Hi,

this library is really simple to use, that would be cool if you can add a function to animate easily GradientLayer (start/pause/stop ?).

[dqhieu]

Hi Relisiuol,

Your idea is very cool but now I don't have much time to investigate animation. I think you can add a function to animate GradientLayer by overriding my class or just create a new one by yourself.
It would be appreciated if you could create a pull request so everyone can use your implementation.

Thanks!