Feature to support multiple shadows
Hi 👋🏻 ,

I'm Roland and recently fell in love with the great work that you're doing here and the endless possibilities that come with it. While already having some PR's running (you've probably noticed), I'd like to request a feature or better discuss the approach to introduce it.

Currently it is only possible to define a single box shadow. I'd like to add a feature to support multiple ones. While sketch already provides setting multiple shadows by passing in shadow styles in an array, the ViewRenderer as well as ImageRenderer do only support resolving the style for one shadow.

https://github.com/airbnb/react-sketchapp/blob/master/src/renderers/ViewRenderer.js#L48
https://github.com/airbnb/react-sketchapp/blob/master/src/renderers/ViewRenderer.js#L104

As a proposal (happy to discuss) I'd suggest to create a style property called `shadowGroup` in parallel. Each of them holding styles for an individual shadow. I'd still keep the current implementation in place for backwards-compatibility. So creating multiple shadows would be an opt-in functionality. 

Let me know what you think. 

Roland