add async attribute to the tag

Can we update the template tag to output with an async attribute? Or if not backwards compatible perhaps a keyword on the template tag 'async' to do so.

http://requirejs.org/docs/start.html

> For browsers that support it, you could also add an async attribute to the script tag.

e.g.

```

```

becomes

```
<script src="static/js/require.a3f22c5ad833.js" data-main="static/js/search.bf074024de9e.js" async defer>
```

https://www.igvita.com/2014/05/20/script-injected-async-scripts-considered-harmful/

Start by locating the template tag that emits the RequireJS script tag and inspect its existing arguments and tests. Decide whether async and defer are default output or opt-in, then verify the generated markup for the requested case while preserving existing output for other callers.

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