Possibility to pull a single folder

### Proposed change

We use nbgitpuller to distribute assignments. We don't want students to get all assignments at once but release them week by week. It would be great if nbgitpuller links could be folder specific so that we can give students a link that will always pull down the latest version of only that assignment.

### Alternative options

Manually uploading only the latest assignments to a separate github repoi, which is what we currently are doing

### Who would use this feature?

Anyone interested in disseminating a partial repo

### (Optional): Suggest a solution

Maybe this git technique could be used? https://stackoverflow.com/a/43902478/2166823