Add support for Gatsby JS

[devraj]

Possibly more relevant for https://github.com/anomaly/use-doko

The general request is to add support for uses in Gatsby JS sites, via Helmet

[brendon1555]

This is really only relevant for the hook, but Gatsby should already work. The hook just needs to be applied to each page. Or placed inside a wrapPageElement component in gatsby-browser and gatsby-ssr. I am yet to test the wrap but the doco suggests thats what we want.
https://www.gatsbyjs.com/docs/reference/config-files/gatsby-browser/#wrapPageElement