This change allows the directory front page to be more easily configurable.
Templates are now read only at start-up and stored in a map rather than re-parsed for each page rendering.
LGTM=adg
R=adg
CC=golang-codereviews
https://golang.org/cl/109080044
This will fix the images in Brad's GoCon presentation.
LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/100950043