Add button to relist/select underpriced items

[Lucki]

A "relist underpriced" and/or "select underpriced" button would be handy which works in the same way as the "relist overpriced" button.

[Nuklon]

Feel free to submit a PR for this.

[disanyixing]

else{
    logConsole('Sell price is fair.');

    if (getSettingWithDefault(SETTING_RELIST_AUTOMATICALLY) == 1) {
        if (histogram.sell_order_graph.length >= 2) {
            let firstsellprice = histogram.sell_order_graph[0][0];
            let secondsellprice = histogram.sell_order_graph[1][0];
            if (secondsellprice - 0.5 > firstsellprice ) {
                queueOverpricedItemListing(listing.listingid);
            }
        }
    }

    $('.market_listing_my_price', listingUI).last().css('background', COLOR_PRICE_FAIR);
    listingUI.addClass('fair');
}

I have added a feature in the above code that automatically relist item when the second selling price is 0.5 higher than the first selling price.