[feature] callbacks/events before import and append.

[tomalec]

Description

I need to access the elements and stub few things precisely when they are created (imported) then when stamped (connected).

Actually, I'm trying to test a polyfill for the declarative shadow dom element, according to latest ideas which consist removing the element once parsed.

Expected outcome

I'd love to have an event or callback:

• imported - with a NodesList of imported nodes as a detail/parameter, so I could access them even though the may get removed from the document after constructor/connectedCallback/fixture. Called just after content of tempalte is imported.
• connected - when the elements are being appended to the main document.

I could provide a PR if that's the idea with any chance to get merged.

Actual outcome

No way to catch those.

Note, that I cannot stub into Custom Elements constructor.

Live Demo

Steps to reproduce

Browsers Affected

• Chrome
• Firefox
• Safari 9
• Safari 8
• Safari 7
• Edge
• IE 11
• IE 10