Renaming instance context, adding ability to get badge instances by email

I'm looking to add an optional comment when issuing a badge that gets passed on to webhooks.  This makes my original choice of name for the "instance" part of the context rather silly (it was "email", which would mean your context segment would be `email: { email: 'blah@example.org", comment: "Blah." }`, which would be real dumb.

Note that despite being a tiny change, this will require a new version, because it will break the old version of createBadgeInstance.