Support multiple entity modification with a single command

[ptitFicus]

In its current state, scribe only allow modification of a single entity with one command.

However there is some event sourcing use case when modification of multiple entity at once is necessary (e.g. bank accounts).

Therefore it would be a good feature to have.

Some rough ideas:

• replace command entityId by a list

• allow command handler to return a list of event with differents entityIds

[ptitFicus]

Currently it's possible to chain command handler with the same transaction, however it makes the code quite cumbersome.

[ptitFicus]

Another idea would be to pass to the CommandHandler a provider that allow to read a given id. The handler could call this provider with every ids it needs.

[ptitFicus]

#41 is for documenting what's currentlty possible, I keep this one open for a more "user friendly" implementation