[webcomponents] Per-type ready event for Custom Elements

We're seeing issues with custom element ready state awareness under various
common async load patterns like AMD, CommonJS, etc. Essentially, when a
developer brings in their definitions via one of these systems, the
DOMComponentsLoaded/WebComponentsReady event has already fired, leaving
them with race conditions. There was previously an event that fired for
_every_ element node when each one was ready, and it was pulled due to
various feasibility issues (which is understandable). The proposal here is
different: fire one event (customelementready?) when all known in-source
elements of a type/name are parsed and ready for interaction, *regardless
of when that occurs*.

The use case here is simple:

Let's say a dev defines a new custom element with document.register() 10
minutes after the page is loaded. Unphased by the fashionably late arrival
of a new custom element definition, the parser crawls through the in-source
elements, augments any matching nodes, and fires a single event when
finished with the lot of them. There would be a property on the event
(tagName, customElementName?) to inform the developer as to what type of
custom element was ready for interaction.

Make sense? Thoughts? (do we already have this covered some other way?)

- Daniel