Add the possibility to publish redis events for laravel-echo whisper()

[timmy1420]

For months I've been searching for a way how to emit whisper events from the client side.
As we all know, in socket.io we can listen to emit events, but I'm missing this feature in laravel-echo.

Here are the changes I made:

• I added the PresenceChannelWhisper publish event to listen to all whisper broadcast client events on the server side.
• Have redis publishing an optional feature. Just like adding the "publishPresence" key, I added "publishWhisper" key to check for whisper events

[jorgecastillodp]

@tlaverdure