David Cook, whose singing career sky rocketed after being named “American Idol” winner last year, has a special female fan he’s been keeping under wraps. She’s Kimberly Johnston, 35, a busty Atlanta model who travels with him as he tours the country. She and Cook, 26, have been con necting since just about the time he won the competition. The lovebirds have been canoo dling at out-of-the-way places late at night after his concerts. She secretly met Cook at the bar of the London Hotel on West 54t