Add CLI option to register subscribers

[Jean85]

This is WIP, do not merge.

I was looking into #1 and I would personally like and need a way to add subscribers through CLI options. So I drafted this PR using sebastianbergmann#4136 as a base. While doing this I noticed two different things:

• there's no way yet to reach the Dispatcher singleton to register new subscribers in any way
• registering the subscribers in the way that I'm trying to is pretty limited, since it's in the TestRunner: it could happen too late for a number of events that are fired before

So... How do you want to proceed? What steps can I take?

[EDIT] /cc @sebastianbergmann, this is my initial feedback on this implementation

[theseer]

This will be done via our Events\Facade.

Right now, you could query the Facade for the Dispatcher (Events\Facade::dispatcher()) and call register() to add subscribers.

This is work in progress and the Facade will soon provide a direct API without leaking the Dispatcher.

[theseer]

Thanks for the work already.

While I'm not yet sure what your goal is, I'm not convinced this is going the right way: A Subscriber is code that needs to be loaded and run. A Subscriber by itself is just a receiver of events. Why would you need a CLI option for that?

We need a technical API to have "things" register subscribers but that step is a prerequisite to actually do something with the event and I don't see how this would be a CLI option?