godoc: specify HTML page metadata with a JSON blob

godoc: specify HTML page metadata with a JSON blob

This allows HTML pages to specify arbitrary data in a header:

<!--{
        "Title": "The page title",
        ...
}-->

replacing the old style comments:

<!-- title The page title -->

R=gri, rsc, r, bradfitz, dsymonds
CC=golang-dev
https://golang.org/cl/5532093