Add spinner animation

[rbalet]

With the before and after logic you've built, it's actually pretty easy to add animation, like a slide-down one.

Here a list of Ideas for animation. https://lab.hakim.se/ladda/

I'll be developing it in my library, will push it here if you think you'll be merging it :)

[dkreider]

@rbalet thank you for the ideas. And please push any helpful changes!