The matrix.eventable module/function returns a table with a pair of functions
which can be used to connect event handlers to events, and to generate them.
Basic usage is as follows:
obj = eventable()
obj.hook("event-name", print) -- Handle the event with "print"
obj.fire("event-name", "Hello, world") -- Prints "Hello, world"
This initial implementation has the following limitations:
* Event handlers are always invoked in the same order in which they have been
connected using the .hook() function.
* Individual event handlers cannot be disconnected. Only disconnecting all
handlers for en event at once is possible via .hook("event", nil).
