Mixpanel name_tag should prefer using email over user ID

[philfreo]

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, which is fine, but I think its perhaps more common 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.

[philfreo]

haven't got the testing env setup locally yet so the build is failing, but hopefully you get the gist of what i'm going for here

[ianstormtaylor]

Yup I gotcha, good call. Merging it in now