The one with variant metafields and pre launch enabled flag

Overview

We've implemented support for metafields on the variant struct

type Variant struct {
	ID                   int64            `json:"id,omitempty"`
        ...
	Metafields           []Metafield      `json:"metafields,omitempty"`
}

and the pre_launch_enabled flag on the shop model.

type Shop struct {
	ID                              int64      `json:"id"`
        ...
	PreLaunchEnabled                bool       `json:"pre_launch_enabled"`
}

#63 Adding pre_launch_enabled to shop model (Thanks @fidel-ruiz)
#60 Add metafields to the shopify variant struct (Thanks @sandersjessica)