Animate when visible (use intersection observer) (#356)

* Update options and README

* Adding demos

* fix orphaned observer issue

* onDestroy method

* more demo refinements

* auto animate options in demos

* modernize demo syntax (RIP var, xmlhttprequest)

* add specs, lint

* build

* update README

* further README improvements

* copilot PR feedback

* add tabular nums blurb in README

adding parse tests, lint, build

bump version

provide exports

bump version, add test for onStartCallback

bump minor verion for updated main in package.json

make rollup not rewrite this to window

Rollup build (#311)

* replace gulp build with rollup

* bump version

* point main to es6 module

* revert change to test umd

* remove browserify

* update eslint config

* fix rollup error

* update readme

Support plugins (#307)

* support plugins

* add plugin test

* Update README

Add onCompleteCallback option (#303)

* add onCompleteCallback option

* update demo

* new tests

* update docs

* update index