Metallics and slip-on shoes are both trending right now. Leave it to Andie MacDowell to combine the two. Ever since she stepped out in shiny silver loafers last month, ‘it’ girls are copying the look for everything from casual errands to fancy dinners. We’re doing the same — for under $50! Metallic loafers are one of this summer’s biggest trends already, and MacDowell practically wrote the rulebook. She strolled into the Cannes Film Festival loo…