allow not using new

[towc]

I was trying to figure out why I had an obscure EventEmitter error, and it took me very long to figure out that I missed out new.

Most of the express ecosystem constructs instances without using new, which is probably why I was confused. I'm not suggesting new is completely dropped as that would break everyone else's code, but that not using new should work as well.

It should be quite straight forward to implement. For plain node, this works:

> function X() { if (this === global) return new X(); this.y = 2 }; [X(), new X()]
[ X { y: 2 }, X { y: 2 } ]

I'm not sure if this would work with bundlers.

If you don't want to allow that behaviour, it would be nice to at least throw an error saying that you didn't use new, instead of resulting in this getting thrown:

events.js:85
  if (this._events === undefined ||
           ^

TypeError: Cannot read property '_events' of undefined
    at EventEmitter.init (events.js:85:12)
    at EventEmitter (events.js:44:21)
    at Store (/app/node_modules/express-session/session/store.js:33:16)
    at KnexStore (/app/node_modules/connect-session-knex/index.js:190:9)
    at Object.<anonymous> (/app/src/app.js:20:24)
    ...