Tree examples: add example of multiple selection

[mcking65]

All the tree examples are single select. Add a multi-select example or update one of the existing examples to use multiple selection. The file browser example is a good candidate for multiple selection.

Preview links for work in progress

Examples are being developed in the add-multi-select-tree branch. The current examples are visible in rawgit at:

1. File Directory Multi-Select Treeview Example Using Declared Properties
2. File Directory Multi-Select Treeview Example Using Computed Properties

[mcking65]

@jongund, This takes us to 6 tree examples. I think we should consider reducing to 3. What if we had:

1. Navigation with computed props
2. single select file browser with declared props
3. multi-select file browser with declared props

Or, something like that ... give some thought. I don't think maintaining 6 example pages is necessary.