Of course, by 'mainstream' I mean that celebrities are now endorsing the campaign. Paul, Stella and Mary McCartney today launched a new website, Meat Free Monday, which encourages everyone to, yes, you guessed it, give up meat on Mondays. Other famous names weighing in include Kevin Spacey, Sheryl Crow, Woody Harrelson, Alec Baldwin, Ricky Gervais and Chris Martin.The idea of cutting back on meat isn't a new one: think of rationing during the wars, due to short supplies. But this time it's out o