ACTION-129: plugin-types inherits into plugin documents

I have ACTION-129 to propose some spec text for inheriting
plugin-types into plugin documents.  I've made a first pass at adding
that to the spec:

https://dvcs.w3.org/hg/content-security-policy/rev/a7c49fc7ca0c

Hixie was kind enough to add a definition of "plugin document" to the
HTML spec so that we could hook into the spec unambiguously.

Please let me know if you have any comments.

Adam