[5.5] An easy way to route notifications on the go

[themsaid]

This PR allows for the following:

(new InstantNotifiable())
            ->addRoute('enzo@deepblue.com', 'mail')
            ->addRoute('https://slack-webhook...', 'slack')
            ->notify(new TestNotification())

Sometimes you need to use an existing notification class to send to a non-existing entity, like a specific slack webhook that doesn't belong to any notifiable, or global email address that doesn't belong to any notifiable.

[martinbean]

YES! This is going to be so handy. I have a load of notifications I send to Slack on the back of events in an application, where I “notified” the admin user(s).