Add a way to inline custom javascript (via Helmet).

[piotrfonte]

Helmet can do:

        {/* multiple script elements */}
        <script src="http://include.com/pathtojs.js" type="text/javascript" />

        {/* inline script elements */}
        <script type="application/ld+json">{`
        {
            "@context": "http://schema.org"
        }
    `}</script>

Lets allow to provide custom js snippets in the app somehow (useful for adding analytics, among other things).