It's hard to overstate Jack Antonoff's influence on pop music: He's written or produced some of the biggest songs with some of the biggest names in the business, while also performing as lead singer of the band Bleachers.Correspondent Tracy Smith talked with the 11-time Grammy-winner about his latest milestones, from a sold-out show at Madison Square Garden to a Broadway production of "Romeo + Juliet," as well as marriage, songwriting, and what Taylor Swift has meant to him....