Add alternate_editor config option to specify an editor.

[bossstein]

Description

Solution to issue #1342 . This change adds an alternate_editor config option. Previous functionality was to check the $VISUAL environment variable and use this and then if this is blank it would use $EDITOR. These environment variables are used by other software and as such a user may not want to change them, however they may want to not use a visual editor in pgcli or they may want to use a different editor to the default. By adding this alternate editor option a user can choose a different editor and even specify arguments if they want to use different editor behaviour while in pgcli.

Checklist

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

[bossstein]

@j-bennet can you check this pr when it's convenient.

[j-bennet]

@bossstein Do you plan to continue working on this PR?