render with only component markup

For server-side rendering, it would be useful to have a render method that only returns markup specified by components, without `data-reactid` attributes and `window.__reactAsyncStatePacket`, like `React.renderToStaticMarkup`.

Using `renderComponentToStringWithAsyncState` gives the warning:

```
Warning: ReactAsync.renderComponentToStringWithAsyncState will be deprecated in a future version. Use ReactAsync.renderToStringAsync instead.
```

Is there a way of achieving this, or one in the works?

Related: #14 #15