exp/template/html: Added handling for URL attributes.

exp/template/html: Added handling for URL attributes.

1. adds a urlPart field to context
2. implements tURL to figure out the URL part
3. modifies joinContext to allow common context mismatches
   around branches to be ignored when not material as in
   <a href="/foo{{if .HasQuery}}?q={{.Query}}{{/if}}">
4. adds a pipeline function that filters dynamically inserted
   protocols to prevent code injection via URLs.

R=nigeltao
CC=golang-dev
https://golang.org/cl/4957041