Add 'minimal' format: the same as 'plain', but without column headers.

[gfrlv]

Description

By popular demand dbcli/mycli#872

Checklist

• I've added this contribution to the CHANGELOG.
• I've added my name to the AUTHORS file (or it's already there).
• I installed pre-commit hooks (pip install pre-commit && pre-commit install), and ran black on my code.
• Please squash merge this pull request (uncheck if you'd like us to merge as multiple commits)

[j-bennet]

Looks good!