Add regolith config command

[Nusiq]

Description

The regolith config command would let the user set various settings to change how Regolith works. This is similar to what git config command does in Git. The configuration should be stored in two places:

• .regolith path for project configuration.
• somewhere in the application data for global configuration (accessed with --global flag)

Note that the .regolith folder is in .gitignore, so every team member working on the same project can have their own project configuration.

The regolith config command is not related to config.json file.

Settings that could be modified by the command

Currently the list of the settings is short. I think the two settings listed below are the most important. I don't see any better ways to implement them than adding an option to Regolith to let the user to have their own application configuration:

• use_app_data - a boolean. This setting should replace the useAppData property from the config.json file. IMO adding that to the config.json was a mistake.
• resolvers - a list of strings with URLs to repositories that could be used as filter resolvers. This would enable teams to link their private repositories.

Other options that we could add:

• user_name - could be used by regolith init to fill the author field.
• default_export_target - overwriting the output path generated by regolith init
• custom properties - perhaps we could expose the settings to the filters. In this case adding any custom property could be used for something.

Implementation details

I'm not sure about the implementation details yet. For example I don't know what format would be the best for the config file (maybe TOML?). I don't know how to support modifications of a list for default_export_target. Maybe adding this command is not necessary. In this case Regolith could just read the config file that the user would have to modify manually.

[Nusiq]

This could be a solution to #176