Mixpanel name_tag should prefer using email over user ID

Typically `userID` is something unfriendly like a number or arbitrary string. In your docs `identity()` example you happen to use their email as the user id, but I think it makes sense for people to pass in their truly unique id (which emails may not be) to `identify()`

Before, mixpanel.name_tag passed the ID -- the change passes the email if possible

From https://mixpanel.com/docs/integration-libraries/javascript-full-api#name_tag it says "human-readable", which an email is, but an ID often isn't.