facebook-ads: add support for custom events

with this `.track` always sends an event:

- if it's in the `events` map, then the event name gets translated and it sends the 2 special conversion event properties (currency and value)
- otherwise, it just sends along the original event name and any properties you might have passed along.

/cc @calvinfo 
maybe just set `data = track.properties()` here to avoid if/else