Added 5 fields

[DLBPointon]

Hi Elaws,

Thanks for producing such a robust script, I've been using it quite a bit over the weekend.

I've added another 5 fields from the API that I quite like to have:

• avRating - Average Rating reported by the API
• genre - the categories field in the API
• bookDesc - The blurb of the book
• mature - A rating for whether or not a book is rated as MATURE or NOT_MATURE
• pageCount - The total number of pages reported for the book

This all works <--

I also tried adding retail information, I like to sum up how much my books have cost me and how much I'll be spending on those I haven't read or bought yet, but I ran into an issue where it just does not exist in many cases.

I tried:

	const selectedBookRetail = selection.saleInfo.retailPrice;

	let gprice = 0
        // Presence of retailPrice.amount indicates presence of currencyCode
	if (typeof(selectedBookRetail.amount) !== "undefined") {
		let getPrice = selectedBookRetail.amount
                let getCurrency = selectedBookRetail.currencyCode
	}
       ......
        currency: getCurrency
        price: getPrice

But I don't know enough JavaScript to make this work completely. But I'll work on that if you don't want to.
Thanks,
DLBPointon