Allow nested object properties for children,label attribute

[adityarj]

Currently the label or children attribute has to be a direct property of the node.

<div ivh-treeview="model" ivh-treeview-label-attribute="'name'" </div>

Being able to access nested object properties would be useful

<div ivh-treeview="model" ivh-treeview-label-attribute="'data.name'" </div>

I'm interested to work on the attributes

[jtrussell]

Sorry, just getting around to some things now. If you'd like to submit a PR with tests we'd be interested.

[adityarj]

Alright, will give it a go