Support for message element?

Hi, thanks for this project. I've struggled with `aria-live` regions in React before so I'm glad this exists. However, I can't use this library because `PropTypes.string` is the only allowable type to the `message` prop.

I need to have an element in the aria-live region. The main reason for this is because I need to wrap the text in a span that indicates the language of the text. E.g.:

```html

Hello, world!

```

This is because I use a translations service for all of the display strings in my application, and sometimes, some of the strings are not translated to the user's language. A screenreader needs to know the language of the string to read it out correctly.

I don't know about the internals of this library, but do you think it would be possible to make a change to support passing elements to the `message` prop (`PropTypes.node` is a great option for this)?