Support multi-location inventory

[bogdanghervan]

Shopify launched a long-awaited feature called multi-location inventory back in May.

The former inventory API is now deprecated and will stop working in July.

After July 1st, apps will no longer be able to set inventory using inventory_quantity or inventory_quantity_adjustment.

More details on the Migrating to multi-location inventory page.

[JelleDijkhuizen]

I thing pull #46 added this functions