I'm not sure if it's a recent feature, but I was unaware of it until recently. However, you'd still need an app to sign the upload URL. Otherwise, you'd have to expose the AWS secret key to the client.
It's mostly a matter of setting up an endpoint in your app to sign manifests using the secret key. It's give or take a dozen lines of code on the backend (to do it in an "unsophisticated way"), and was one of the first things I wrote in Elixir! These two links should get you on your way:
1. http://bencoe.tumblr.com/post/30685403088/browser-side-amazo...
2. https://aws.amazon.com/articles/1434/?tag=vig-20