Add custom styles for auto-generated divs with animations

[Sebight]

I used this library in my project, but had problems with implementing it with my current structure.

I've written myself a small tweak, which allowed me to apply custom styles to the automatically generated. It is simply sent as a prop and then applied in the component.

This can come in handy in many cases and does not change the original functionality of the library.

[shiryel]

Hi Sebight, I appreciate the PR, but I would like to know if it were not possible to use the css_observer and the css_animation external variables to accomplish the same result. Their initial objective were to customize the styles by the parent, they are not used internally by the lib in any other way.

[Sebight]

Oof. Haven't actually noticed that.

That might be enough though, yes. But if you ever find this modification handy, feel free to merge it! :D