Enable support for the assetsPath option

Hey 👋 

It would be handy to add support for the [assetsPath](https://github.com/airbnb/lottie-web/wiki/loadAnimation-options#assetspath) option so it's possible to [change the base path](https://github.com/airbnb/lottie-web/issues/284#issuecomment-376355553) of external assets.

Thanks!