[TODO] Implement a mechanism for synthetic events

[webknjaz]

There's some commonly needed events that are combos of a few conditions. One example is PR "merged", GitHub does not emit such an event, it does not exist. But we could make it easier for the end-users to subscribe to it.

The idea is to synthetize fake events that would essentially implement pre-processing of any incoming events and so that the user-defined event handlers are only invoked under certain conditions. Checking the merged PR means checking two flags: closed and merged (because some closed PRs are unmerged).

Users can have a boilerplate for such event but it'd be cleaner to subscribe to a synthetic pull_request.merged event.

I think implementing this feature may also help us implement #36 using the same technique as it would be a synthetic event matching anything too.

[webknjaz]

Possible extra use case: https://github.com/thoth-station/qeb-hwt/blob/19d3d3f/app.py#L156-L157