Adds `expandedNodes` method to the tree.

[nathanbowser]

This returns all the nodes that are expanded.

Fixes #448

[nathanbowser]

@jerryorr This is part one of your request from our slack chat early. Do you want the ability to set some state on the incoming nodes so the tree is rendered correctly, or do you want to render and then expand each of these nodes you persisted?

[mattsgarlata]

@nathanbowser for a very large tree, I would think manually expanding the nodes would be really slow, wouldn't it?

[nathanbowser]

Yea good point. I'll add support for initial renders because we already performance tune the initial drawing.

[nathanbowser]

Updated.

@jerryorr You can set collapsed: false on any of the persisted nodes that should be expanded on initial render.

I am thinking collapsed: false might seem weird, but that follows the internal conventions used by the tree, so I stuck with it.

[mattsgarlata]

+1