Add format_timestamps preprocessor

[dougharris]

Description

Added format_timestamps() to preprocessors.

This allows for per-column formatting for date, time, or datetime like data.

Add a column_date_formats section to your config file with separate lines for each column that you'd like to specify a format using name=format. Use standard Python strftime formatting strings.

This works with a related pgcli PR to address my enhancement request dbcli/pgcli#1402

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)