OAKLAND — A planned facility that will convert rubbish to natural gas gained more support Wednesday, when the Town Council voted unanimously to authorize the town manager to sign a 15-year agreement to send Oakland’s trash to the gas plant in Hampden starting in 2018. The proposed plant is a joint project of Maryland-based company Fiberight and the Municipal Review Committee, a nonprofit that represents more than 180 Maine communities that now deliver municipal waste to an power-generating incinerator in Orrington. Oakland is the latest community to sign on to the proposal.