Add support for 'screenview' event

[bblanchon]

This PR adds support for "screenview" events. See https://developers.google.com/analytics/devguides/collection/gtagjs/screens https://developers.google.com/analytics/devguides/collection/analyticsjs/screens

In the CSS, I saw more references to .beacon--pageview. I was not able to fix them, but it's probably a piece of cake for a CSS wizard.

[keithclark]

Thanks for this. I currently working on the next version of GA Debugger so I'll include your PR in the next version.