Add tests for new `event-valuechange` elements

* Tests for subscriber and delegation on <select> elements
* Tests for subscriber and delegation on [contenteditable="true"]
  elements.
Implement new element support in event-valuechange

* Supports <select> elements with different <option> choices
* Supports all elements with [contenteditable="true"]

Maintains performance by accessing raw DOM properties (that
are supported in all browsers).
Add new element manual tests for event-valuechange

* Added tests for <select> elements with <option> choices
* Added tests for [contenteditable="true"] elements
Add caching and replace `getHTML` with `innerHTML`

* Caches `tagName` and `isEditable` inside of the node
* Uses `tagName` to determine how to extract the `newVal`
* Replaces all `getHTML` instances with `innerHTML`
Made changes based on @ezequiel's comments

* Used standards-based `nodeName` instead of `tagName`
* Used better boolean statement in `_poll`
Clarify docs to explain initially unadded elements
Adds extra element info to API and user docs
Merge branch 'valuechange-enhancement' into dev-3.x
Build event-valuechange