Implement process managers as first-class citizens

[nbarnwell]

Current implementations of process managers are up to the consumer of the library. It would be helpful if process managers were actually implemented in Regalo, particularly if Regalo could handle the situation where certain aggregate instances (i.e. by type and ID) could subscribe to certain "external" events emitted by other aggregate instances.

This would remove the need for dedicated event handlers just to tell one aggregate about events that happened in another.