add initial ChurnBee integration

[ChurnBee](https://churnbee.com).

it supports only a handful of [events](https://churnbee.com/docs), the [client](https://api.churnbee.com/cb.js) basically ignores events that are not supported.

clients can send "custom" data using `.custom: { ... }`, should do something there ? i feel like we can just copy non supported properties and put them in `.custom`.

i'm 50% we need to map some properties, still investigating :)

![](https://i.cloudup.com/PEnBsGQHOD.png)