Add support for gtag

[abergs]

Analytics now promotes using the gtag.js API.

Add a forceSyntax/fallback that does this:

//window[_ga]('send', 'event', 'Videos', state, videoUrl);
gtag('event', 'play', {
  'event_category': 'Videos',
  'event_label': videoUrl,
  'event_action': state
});