Support "folder view"

[ElDeveloper]

Current Behavior
Like in nbviewer when you specify a git repo you can navigate through the notebooks contained in folders and through the folder structure.

References

1. For an example, see this link.
2. I'm unclear if this is the same issue as Toolbar should be able to handle multiple index files in a visualization #16, not very familiar with the project's terminology.

[ebolyen]

Let's do this in the next iteration for QZA files.