Use native EventTarget instead of a custom implementation

[kevinhikaruevans]

Is there a reason that this library is using its own implementation of EventTarget, here: https://github.com/pladaria/reconnecting-websocket/blob/master/reconnecting-websocket.ts#L268? Why not just extend from EventTarget?

It would be nice to not be confined to the events in events.ts, so I could dispatch other events.