This does not wire up <style> elements as that is pending support
for raw text content in CL https://golang.org/cl/4964045/

This CL allows actions to appear in contexts like

selectors:        {{.Tag}}{{.Class}}{{.Id}}
property names:   border-{{.BidiLeadingEdge}}
property values:  color: {{.Color}}
strings:          font-family: "{{font-name}}"
URL strings:      background: "/foo?image={{.ImgQuery}}"
URL literals:     background: url("{{.Image}}")

but disallows actions inside CSS comments and disallows
embedding of JS in CSS entirely.

It is based on the CSS3 lexical grammar with affordances for
common browser extensions including line comments.

R=nigeltao
CC=golang-dev
https://golang.org/cl/4968058