This addresses several use cases:

(1) <h{{.HeaderLevel}}> used to build hierarchical documents.
(2) <input on{{.EventType}}=...> used in widgets.
(3) <div {{" dir=ltr"}}> used to embed bidi-hints.

It also makes sure that we treat the two templates below the same:

<img src={{if .Avatar}}"{{.Avatar}}"{{else}}"anonymous.png"{{end}}>
<img src="{{if .Avatar}}{{.Avatar}}{{else}}anonymous.png{{end}}">

This splits up tTag into a number of sub-states and adds testcases.

R=nigeltao
CC=golang-dev
https://golang.org/cl/5043042