Doesn't work outside of DOMContentLoaded

```js
import 'date-input-polyfill';
```
```html
<script src="app.js"></script>
```
```
TypeError: null is not an object (evaluating 'document.body.appendChild')
```

Likely caused by this:
https://github.com/jcgertig/date-input-polyfill/blob/master/date-input-polyfill.js#L21

Will you accept a PR? I see that there're unmerged PRs.