Add custom git-config vars for student(s) pairing

Heads up @quackingduck.

This is setup for a custom branch command that will use data from `set-git-user` to enforce a branch name format. This will allow an alias such as the following:

```
[alias]
  challenge-branch = "!git checkout -b \"challenge|$(git config --get-all dbc.student | paste -sd"," -)\""
```
e.g.
```
> weare mattbaker quackingduck
> git challenge-branch
Switched to a new branch 'challenge|mattbaker,quackingduck'
```

**Motivation**
We'd like to be able to write scripts that track student submissions. One approach to "tagging" a PR is to enforce a branch naming format that we can parse and work with via GitHub's API. 

Open to feedback, as always.