Allow multiple emails per user

[kurucu]

Users often have multiple email addresses, for example for work and home; as well as perhaps a private one for GitHub. I don't think identity should be coupled tightly to email address, the relationship should be 1:many, and it would be great if Authorizer supported this.

Allow users to sign up with a first e-mail, and then verify any number of unique email addresses after that. Emails being added would be unique amongst verified email addresses across the whole user set. Verified email addresses would be sent as an array with the user object when requested. A user-selectable "default" one might be sent in place of the current "email" property.

Alternatives are that each user needs separate accounts for work and home, and duplicates are accidentally created with signing in with GitHub etc if they use different (e.g. private or masked) email addresses.