Add plugin hook to execute commands before git pulling

### Proposed change

We write a [pluggy](https://pluggy.readthedocs.io/en/latest/) based plugin system for nbgitpuller. The first hook would allow arbitrary command execution before the git-pulling happens.

This enables use cases where you want to pull from something *other than git*. But you still need git for the diffing and merging capabilities. So the solution is to:

1. Pull from original source (such as canvas, dropbox, blackboard, google drive, etc) into a hidden git repository
2. Then do nbgitpuller things from the hidden repository

This gives us proper merge resolution, without having to implement that ourselves.

### Alternative options

Support pulling from alternate sources such as canvas, URLs, blackboard, etc natively. This could provide a better integrated experience, but at the cost of much higher maintenance burden. We will also have to provide diffing and merging implementations without git, which is hard.

### Who would use this feature?

People who want to use solutions other than git to store their content. [This discourse post](https://discourse.jupyter.org/t/repo2dockerspawner-alternative-version/3584/15?u=yuvipanda) outlines some uses.

### (Optional): Suggest a solution

[rclone](https://rclone.org/) lets you sync files locally from many sources - dropbox, google drive, ftp, etc. We could try get that to work for a single source with rclone, and see what plugins we need to provide.