Allow user-specific configuration

[sunshowers]

Originally came up in #1348 -- at some point we'll likely need to grow a way to do user-specific configuration, not just repository-specific configuration.

Some considerations:

• Location of config
  • XDG everywhere? What about Windows? https://docs.rs/etcetera can help.
  • Allow config directly in home dir as a fallback?
  • Look at what jj and atuin do
• Which options should we let users set?
  • Hiding the progress bar as in [nextest-runner] add option hide-progress-bar to configuration file #1348
  • Other output-related options are also easy -- e.g. status-level, final-status-level
• What if repository and user configs clash?
  • Probably want repository config to generally override user config, but also let users say they want to override repository config
  • Current method uses environment variables, which always override repository config!