Add custom elements to HTML

This upstreams custom elements into the HTML Standard, instead of having
them be in a separate spec that monkeypatches HTML extensively. This
builds on previous work in whatwg/dom#204.

See https://rawgit.com/w3c/webcomponents/15a203c8393aef0df7223ab1d43406aa11a7e71e/spec/custom/index.html
for the source material, in particular the sections labeled "HTML+".
Most changes while upstreaming were editorial in nature, with the
following exceptions:

- Changes were made to the "element interface" section to nail down
  underspecified edge cases, and fix https://www.w3.org/Bugs/Public/show_bug.cgi?id=27877.
- Changes were made to the authoring conformance criteria for attributes
  on custom elements.

A notable change included in this upstreaming is that the order of
attributes created by the parser is now well-defined. See
WICG/webcomponents#474 for discussion, as well
as https://www.w3.org/Bugs/Public/show_bug.cgi?id=17871.

Author: domenic