Adds a rubocop rule (with autocorrect) to ensure line break after guard clauses.
`Project#inspect` isn't very useful for examining usage of these features.
- allows unsubscription processing of email in format "reply+%{key}+unsubscribe@acme.com" (example) - if config.address includes %{key} and replies are enabled every unsubscriable message will include mailto: link in its List-Unsubscribe header
