Not compatible with SSR

Well, this library is not SSR compatible, and there are only two reasons why not:
- `README` tells to add it to the `head` or `bundle`, while it has to be inlined just after `body` tag
- it uses `onLoad/onError` handlers which are __async__

## Let's make it work right!
Long story short - here is working sandbox - https://codesandbox.io/s/crazy-ganguly-hmhe8

- it __inlines__ `base64 img` before the body to tell the browser about it. The `img` is __invisible__.
- it "checks" `img` synchronously after creating, and it does work! (only with a real `img` tag, preload does not help).
- so it sets class to the body __here and now__, and the following HTML would use `webp`
- as long as dataURI inlined in the same document - it does not affect gzip size.

## Tested
In `Chrome`(webp ✅), `FireFox`(webp ✅), and `Safari`(no webp, as expected)

## Outcome

This library becomes SSR friendly!

Happy to open PR with a more compact version of the sandboxed example, however, the main changes are expected at the README level