MICHELLE OBAMA is already hammering away at a $15 million memoir that will rip the lid off President Barack Obama’s secret extramarital hijinks and public flirtations with other women, sources tell The ENQUIRER. “Michelle has suffered in silence, while Barack ignored her to live it up surrounded by adoring beauties,” an insider told The ENQUIRER. “She’s fed up, and she’s going to use a poison-pen memoir of her White House years to get revenge on him