Support data-* attributes.

[JLCarveth]

I think this library should support adding data- attributes to elements. For example, to install Plausible Analytics to a website, it requires a <script> tag with a custom data-domain attribute:

```
<script defer data-domain="domain1.com,domain2.com,domain3.com" src="https://plausible.io/js/script.js"></script>
```

I would like to create a PR for this feature if it is welcome.