html: parse <xmp> tags

Pass tests5.dat, test 10:
<p><xmp></xmp>

| <html>
|   <head>
|   <body>
|     <p>
|     <xmp>

Also pass the remaining tests in tests5.dat.

R=nigeltao
CC=golang-dev
https://golang.org/cl/5440062
4 files changed