Added new Go client to the list

[matryer]

This client solves the common problems of making queries, setting variables, and uploading files, while leaving the rest to the user. It's deliberately very simple, which means it will soften the learning curve for somebody trying GraphQL for the first time.

• See the online Godoc documentation for a sense of its simplicity

Code example:

// create a client (safe to share across requests)
client := graphql.NewClient("https://machinebox.io/graphql")

// make a request
req := graphql.NewRequest(`
    query ($key: String!) {
        items (id:$key) {
            field1
            field2
            field3
        }
    }
`)

// set any variables
req.Var("key", "value")

// run it and capture the response
var respData ResponseStruct
if err := client.Run(ctx, req, &respData); err != nil {
    log.Fatal(err)
}

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