Allow to pass argument on subscription time.

Until now, we could only pass arguments to an event hook when the event
was called
doEvent($name, $args)

Now we can pass the argument at the moment we subscribe to the event:
subscribeToEvent($name, $action, $args...)

This comes handy when we need to delay subscription to the moment when
we have the data the event will use.
For example: if we want to dynamically populate the meta-descriptions
field when the page is created, we will only know which meta-description
to use when the page is requested.