refs https://github.com/TryGhost/Team/issues/1277
- This will allow to filter events within `getEventTimeline`
- The subset of NQL has the following rules:
- Only one level of filters, now parenthesis allowed
- Only three filter keys allowed
- No `or` allowed outside of the bracket notation (this is allowed: `type:-[email_opened_event,email_failed_event]` but this isn't: `type:1,data.created_at:1`)
- The return is an object with a NQL filter by allowed filter key
