Add --regenerate-todo command line option to rebuild .rubocop_todo.yml using its previous options

[dvandersluis]

Added a new command line option --regenerate-todo which regenerates .rubocop_todo.yml using the options used the last time (extracted from the comment at the top of the todo file), or act as --auto-gen-config if there is no current valid todo file.

At my work, we frequently re-run rubocop --auto-gen-config with various options in order to keep the todo file small and relevant. Each time, we have to copy the command from the previous version of the file in order to have it use the same options. Sometimes this is accidentally missed, which results in the todo file completely changing due to the changed options.

With this change I'm trying to prevent that, by just having rubocop reuse its previous config generation options when requested by just running rubocop --regenerate-todo.

rubocop --auto-gen-config --no-auto-gen-timestamp --exclude-limit 1000

# ... later ...

rubocop --regenerate-todo
# uses exclude_limit: 1000, auto_gen_timestamp: false

I also made it so that it's possible to purposefully override the options with --regenerate-todo by passing them in as well. To facilitate this, I changed --no-offense-counts, --auto-gen-only-exclude, and --no-auto-gen-timestamp to have both positive and negative variants, so this will work for example:

rubocop --regenerate-todo --auto-gen-timestamp
# uses exclude_limit: 1000, auto_gen_timestamp: true

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• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
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• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.

[bbatsov]

I love this change! Great work! 🙇‍♂️