Display the abbreviation for each author in the Author List

Display the abbreviation the system uses for each author in the output of `git pair`:

Perhaps something like:
```
Current config:
  Email template:
    Author list: Bob McKenzie <bob.mckenzie@example.com> (abbr: BM)
                 Doug McKenzie <doug.mckenzie@example.com> (abbr: DM)

Current author: Arthur Dent
  Current email: arthur.dent@example.com (abbr: AD)
```