encoding/htmlindex: implementation of htmlindex

Note that the LanguageDefault uses the matcher. This is not strictly
according to spec, but really makes sense and will result in more
useful results.

Change-Id: Ie71579ef41da8e738c16172729e922f3254a244e
Reviewed-on: https://go-review.googlesource.com/10570
Reviewed-by: Nigel Tao <nigeltao@golang.org>
5 files changed