Change typing to have EventEmitter2 extends from EventEmitter

[unional]

Currently EventEmitter2 can't be assigned directly to a EventEmitter interface:

import events = require('events')
import { EventEmitter2, Listener } from 'eventemitter2'

let e: events.EventEmitter
let e2 = new EventEmitter2()
e = e2; // error, can't assign because addListener function is not compatiable

type addListener = (event: string | symbol, listener: Function) => events.EventEmitter
type addListener2 = (event: string, listener: Listener) => EventEmitter2

let l1: addListener = ...
let l2: addListener2
y = x; // same error

Makes EventEmitter2 implements events.EventEmitter could fix this issue.

declare class EventEmitter3 implements events.EventEmitter {
  addListener(event: string | symbol, listener: Function): this
}

let e3 = new EventEmitter3
e = e3; // no error

[RangerMauve]

Typescript hasn't been getting much love by us maintainers, but a pull request for this change would be welcome. :D

[unional]

I can do that. Do you mind I add typescript as a devDeps so I can add a few tests?

[RangerMauve]

Sure!