Add code to get App URL to shopify-api-node

[byrichardpowell]

Given the Shopify context, and the request, return the URL to the app. See this code here.

This may look something like:

const adminPath =  Shopify.Utils.getAdminAppURL(req)
res.redirect(adminPath)

[byrichardpowell]

https://github.com/Shopify/shopify-api-node/pull/419

[mkevinosullivan]

Fixed in #419