There’s nothing particularly glamorous about the world Thomas Allain creates in “Plastic Cup.” Someone is slurring. Someone is stumbling. Someone has relapsed after two months sober. Friends have disappeared. A bed has become a place of retreat. And the person narrating it all is caught somewhere between anger, grief and the desperate hope that the person they care about will find their way back. It’s an uncomfortable place to write a song from and that’s exactly what makes “