Add an EmailAddress entity
Encode email address entity's address attribute when encoding vCards
Decode email addresses as entity instances when decoding vCards
Fix email addresses in the output of ppl ls
Fix email address formatting in ppl show
Fix email address output in ppl email's full address book output
Fix email address output in ppl email's single contact list
Fix ppl email to work with the new EmailAddress entity
Fix the mutt command to work with the new EmailAddress entity
Fix the mutt output formatter to handle Ppl::Entity::EmailAddress
Add an EmailAddress service
Ditch the email command's inheritance from Ppl::Command::Attribute
Put the service layer in charge of adding new email addresses
Put the service layer in charge of removing email addresses
Add the email address service to the dependency injection container
Fix the build: commands still have to return a boolean value

Eventually I need to get around to removing this annoying boilerplate
requirement. Design mistake on my part.
Add "preferred" attribute to Ppl::Entity::EmailAddress
Fix typo
Output preferred email addresses with a star
Decode vCards' EMAIL fields' PREF field as Ppl::Entity::EmailAddress#preferred