The K-cup that sparks so many millions of coffee drinkers to life each morning is appealing to eco-conscious consumers — just as the market for its Cup of Joe appears to be cooling. Keurig Green Mountain said it plans by 2020 to change the plastic composition in the billions of K-cup single-serving coffee containers it sells annually, making them more lucrative to recyclers while removing one of the nagging complaints that the little pods are piling up in landfills. “Our goal is 100 percent Keurig K-cup pods diverted from landfills by curbside recycling,” said Monique Oxender, the coffee brewer’s chief sustainability officer.