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
(Fixes missing tags, extra tags, missing attributes, and such found by tidy -e) R=golang-dev, adg1, adg CC=golang-dev https://golang.org/cl/2039043
R=rsc, r CC=golang-dev https://golang.org/cl/1238044
