Decouple gatsby-source-shopify

[beamercola]

Here's a go at it! Wasn't sure what to write in the docs but this worked for me!

[netlify[bot]]

Deploy request for gatsby-theme-shopify-manager pending review.

Review with commit df5f52f80375402d7fe57946d959a8f1779f28a6

https://app.netlify.com/sites/gatsby-theme-shopify-manager/deploys

[thetrevorharmon]

Closing in favor of https://github.com/thetrevorharmon/gatsby-theme-shopify-manager/pull/77