Add suport to handle message reads event about stealth

[mgomes]

@luizcarvalho I checked out your closed PR.. it makes sense to me, actually. I'd hate to keep adding in more Facebook-specific code to Stealth as we've already done with payload.

The only downside to your existing approach seems to be that we pollute the payload namespace with built-in payloads. But what if maybe we just came up with a naming convention to account for that? For example, maybe all built-in payloads start with _, and so the read payload would be _read or something like that. This way too we can add support for all of the other webhooks Facebook supports (https://developers.facebook.com/docs/messenger-platform/webhook).

What do you think?

[luizcarvalho]

I think this may work, I'll try to use this convention to create a new PR =)

thanks @mgomes

[mgomes]

Closing this once since it's been merged in stealth-facebook. 👍