SERIES PREMIERE: This new music competition series could be an instant hit, a slow roll, or an equally instant flop. Like American Idol and The Voice, it has aspiring talents performing in front of a panel of attention-stealing judges. The slight twist, this time around, is that the contestants are songwriters, not merely singers – so the songs they sing are their own, pitched to the week’s guest artist. For the premiere, that guest artist is John Legend, fresh (or relatively fresh)