Mixpanel/page

depends on https://github.com/segmentio/facade/pull/52

cc @calvinfo 
what do you think of adding a check to settings to make sure it mirrors the behavior on the client-side? https://github.com/segmentio/analytics.js-integrations/blob/master/lib/mixpanel/index.js#L82-L94

I could see making all `.page()` calls since people really have to opt-in to make them on the server-side. on the other hand, when our APIs have diverged it's created a ton of problems, so I'm generally in favor of keeping them the same and respecting the same options.

only since I'm not sure mixpanel is designed to track all those pageviews.