Navigation tree is working

* Can now do
```python
navigate(menu_tree, 'automate_explorer') #goes to automate/Explorer page
```
* Starting points are supported
* Right now the whole tree is just in header_menu
* Up next
 * add other destinations
 * add ability to accept arguments
 * add grafting to assemble multiple subtrees
 * add a partial that closes over the "official" nav tree so it doesn't
   have to be specified