Add triggers for first-visible, long-view and other events to amp-analytics

[rudygalfi]

Offshoot from #871.

There's currently support for triggers of visible and click. There's planned support for tap, hidden, and timer as part of #871.

This request covers adding support for additional triggers that may be helpful, for instance:

• first-visible would fire when an element first becomes visible on screen
• long-view would use some definition of a long-view and fire when an element's visibility meets the criteria.

cc @avimehta @btownsend

[rudygalfi]

I think everything proposed here can be covered by the visibility triggers. If not, then we should move forward with a clean issue to track that outlines a specific use case.