We urban loyalists love to cry foul when we see our idea of the quintessential exurban commuter: affluent and white in massive SUVs, driving farther and farther out I-66 and I-270 in search of cheaper square footage and a bigger backyard. More than a few articles have wondered aloud whether a mini-mansion in exurbia is really a good thing. But the Post’s Alec MacGillis chipped at our preconceptions yesterday by showing us a different kind…