Also add a new rendering mode, and clean up the plain text one.
Then use it to add an info page to to the server in debug mode.
Change-Id: Ifa66a75260965d0e46e874200203ebbc4490e424
Reviewed-on: https://go-review.googlesource.com/c/tools/+/179497
Reviewed-by: Rebecca Stambler <rstambler@golang.org>
