Feat: GraphQL Fragments

[moonmeister]

Gatsby has a not so well known ability for plugins to export fragments via a fragments.js file. e.g. https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-transformer-sharp/src/fragments.js

It'd be cool to make those available for folks so they don't have to write massive queries for SEO.

Might do a PR if I get time.

[ashhitch]

@moonmeister this would be a great idea!