Events registration

@andrzejkrzywda proposed the idea of registering events in RES. Syntax and API not known. Let's brainstorm.

```ruby
class OrderPlaced
end

event_store.register(OrderPlaced) do |order_placed|
  # order_placed would be a proxy or something like it...
  order_placed.subscribe(to: [OrderSummaryHandler])
end
```

The benefit could be that RES would know about all possible events.