Promise based Queue, written in ES6, works on Node v6+

• import Queue from 'promise-queue-es6'
• new Queue(Number maxConcurrent, Number maxQueued): Queue max numbers are default to Infinity, means unlimited
• Queue#add(Function generator): Promise - adds function argument that generates a promise to the queue`
• Queue#pendingLength - read only property to get number of current pending promises
• Queue#queueLength - read only property to get number of current queued promises