Shadow DOM spec for v1 is ready to be reviewed

Let me post a quick update for the Shadow DOM spec:
https://w3c.github.io/webcomponents/spec/shadow/

I've almost done the spec work for Shadow DOM v1. I think it's time to be
reviewed and get feedback. I hope that a browser vendor, including me, can
start to implement it based on the current spec.

You might want to use https://github.com/w3c/webcomponents/issues/289 to
give me feedback. Please feel free to file a new issue if preferred.

What's changed?
- Looks at the commits in the last month [1]. Most changes are for the
*slots* proposal. I think the current spec has a processing equivalent to
the proposal, that's my intention.

Disclaimer:
- There are still issues labeled with v1 [2]. Most of them are unrelated to
the conclusion of f2f.
- Some of the remaining issues are difficult to address in the Shadow DOM
spec because it requires non-trivial monkey patches to DOM.  I have a plan
to upstream the Shadow DOM spec into DOM in the near future. After that, I
think we can address some of issues by updating DOM spec directly.

- [1] https://github.com/w3c/webcomponents/commits/gh-pages/spec/shadow/
- [2] https://github.com/w3c/webcomponents/labels/v1