Added shorthand methods and 3rd Array args

I just realize that it might have been better to have a different order
in the array i.e. eventType first. However, I'd rather write [ true ]
than [ 'value', true, false ].
Some combinations won't be so common or allowed, so I think this will be fine.

To be able to have a .once() ability would be very nice for multiplayer games
or large indexes of fast changing data, where you wouldn't wan't to
update every ms, but rather every few seconds.

And a an ability to specify the eventType is something I missed...

I've made sure that binding to props is not allowed, as it is bad practice. But it is nice to have the ability to retrieve some props from a remote firebase Index, for initial loading. For example: resuming a session after mounting a React component.