Support Pusher Auth Endpoint

[ngs]

Added Auth Endpoint support.

How to use

pusher_client = new PusherClient
  key: (process.env.PUSHER_KEY or pusher_key)
  authEndpoint: 'https://myapi.mydomain.com/auth/pusher'

or

pusher_client = new PusherClient
  key: (process.env.PUSHER_KEY or pusher_key)
  auth:
    # Any of request npm module options: https://git.io/vgBTa
    url: 'https://myapi.mydomain.com/auth/pusher'
    headers:
      'X-CSRF-Token': 'SOME_CSRF_TOKEN'