Support Facebook messaging_referral events

[johngriffin]

This new feature for Facebook Messenger allows you to pass a referral parameter into a m.me URL. This param is sent to the bot using the "messaging_referral" webhook, which is not currently supported by Botkit.

More info on the feature from Facebook:
https://developers.facebook.com/docs/messenger-platform/referral-params

It seems that we need to recognise this event in /lib/Facebook.js and create a corresponding event in botkit, to be used:

controller.on('facebook_referral', function(bot, message) {
   console.log('the referral code is' + message.referral.ref)
});