"Engine" mode for JS engines that implement async & await

• Add 'engine' mode that only transpiles the nodent ES7 extensions (async get x(){}, async throw x, async return and await anywhere). Standard ES7 async/await constructs are passed through the compiler unchanged to be run by the underlying engine (e.g. Chrome v53 or Edge v14).
• Implement parsing for proposed ES7 Object spread var {a,b,...x} = y ; used in Babel (see MatAtBread/fast-async#6)