Package Gatsby Helpers

[ErisDS]

I've added the code for a gatsby (react, but uses gatbsy-link, not react-link) version of the Tag helper here:

https://github.com/TryGhost/Ghost-SDK/tree/master/packages/helpers-gatsby/src

This needs to be packaged and built into helpers-gatsby/Tags.js so that it's possible to do

import Tags from @tryghost/helpers-gatsby

This will be used in our docs and other gatsby projects e.g. here.

I think a good package to model this on might be https://github.com/nfl/react-helmet, but I'm not 100%.