async await middleware

[원본](https://www.acuriousanimal.com/blog/20180315/express-async-middleware)

const asyncHandler = fn => (req, res, next) =>
  Promise
    .resolve(fn(req, res, next))
    .catch(next)
    
app.get('/hello', asyncHandler( (req, res, next) => {
  // Some code here. Any error will be catch and pass to expressjs
}) );