xml: add Escape, copied from template.HTMLEscape.
xml: add Escape, copied from template.HTMLEscape.

R=rsc
CC=golang-dev
http://codereview.appspot.com/186282

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

tav/go,tav/go,tav/go,tav/go,tav/go,tav/go,tav/go