Are Booming blade and Green-flame blade part of the attack action?

@mikemearls @JeremyECrawford are booming blade and green-flame blade part of the attack action, or do they replace it? It's worded vaguely.

— Nathaemius Varrowe (@dmvarrowe69) January 30, 2016

You use the Cast a Spell action to cast a spell, including a cantrip. #DnD https://t.co/L4KG2az7Bt

— Jeremy Crawford (@JeremyECrawford) January 30, 2016