These expose the main HTML rendering functions of this package
for use with alternate HTTP muxes and on alternate paths.

Fixes golang/go#12195.

Change-Id: I679583fd26116bc83ff551a5d2a1d73ffa1e01f0
Reviewed-on: https://go-review.googlesource.com/13825Reviewed-by: Dave Day <djd@golang.org>