add customerio page support

customer.io's js lib calls `.page` on load, but they also have an method by which we can report a virtual pageview on the client side. 

It takes the url and then an object containing window width, height, and the referrer. Here's the email from their team explaining: https://segment.zendesk.com/agent/tickets/18002

And here's an example of it being called in their lib: https://cloudup.com/cKAB8hbsVoT