script type="module" support

Custom JavaScript can be loaded in `metadata.json` like this:
```json
{
    "extra_js_urls": [
        {
            "url": "https://code.jquery.com/jquery-3.2.1.slim.min.js",
            "sri": "sha256-k2WSCIexGzOj3Euiig+TlR8gA0EmPjuc79OEeY5L45g="
        }
    ]
}
```
Add a `"module": true` option which causes the resulting script element to use `<script type="module">`