Add verify-emails to allow mutating in mass

Created by: mdaniel

There seemed to be precedent for using subshells to generate flag values, which was especially relevant for this since we need a list of tuples, so a JSON structure seemed like a reasonable choice.

fixes: #17

cc: @nicksnyder (this is the last one I had on my "I think I can do that" list, and hopefully we're getting to a pretty representative sample; it's my first time writing GraphQL, so be gentle 😁 )