Michelle Obama has pushed Americans to eat healthier and to exercise more. Now she says we should “drink up” too. As in plain water. And as in more of it. The first lady, an exercise fanatic who loves French fries and whose biceps are envied by women everywhere, is getting behind a campaign being launched Thursday to encourage people to drink more plain old-fashioned water. Whether it comes from a faucet, an underground spring, a rambling river or a plastic bottle, the message is: “Drink up.” …