Ability to initialize the tree with focus a specified node

[tokyoben]

A way to focus on a node automatically when creating the tree. This property should also trigger the node-focus event of the tree and possibly expand all parents although this is currently supported via the state: {"opened": true} property.

[N00ts]

This is a good improvment, I will unfortunatly not have time to implement it soon. If you cant you can create a PR then I will check it :)