[WIP] major reorg of custom elements

- Figured out where custom elements will go in HTML, and started writing as if they were there
- Fleshed out and moved around examples and motivation to form an "introduction" section
- Separated out and rearranged concepts a bit better (fixes #437)
- Started the process of moving the registry to the browsing context instead of document (#369)
- Moved to `window.customElements`, an instance of `CustomElementsRegistry`, with one `define` method currently (#431)

Still to-do:

- fix all broken links detected by respec; a few terms have changed
- Look up registry on the browsing context instead of the document
- Move custom element semantics into the "HTML+: Custom elements" section
- Define a HTML+ patch that adds a `customElements` property to `Window`.