Make (M-x tidy-imports) prompt for command line options

[sflicht]

This lets the user override the script defaults (if desired)

[sflicht]

It would also be cool if we could add a [tool.pyflyby] section to a directory-local config file (e.g. pyproject.toml) to override the default options for tidy-imports (and other scripts), perhaps defaulting to a configuration in the user's .pyflyby directory. That seems like a more involved refactoring job.