[Request] Get an `is_fullscreen` property for container/node.

[ChristianJacobsen]

Hi,

really like the library so far. Nicer for me to use than the python version.
But there's one feature that I'm really missing, and that is knowing if a container/node is currently fullscreen or not. This one can be optional if i3 does not always provide this, but it would be very helpful to be able to get the value.