html/atom: sync html table with the current whatwg spec

Some elements and attributes have been removed from the spec,
but are kept here for backwards compatibility.

Also, this makes gen.go support go:generate and write contents into
target files directly.

Finally, this makes `go generate` work on an entire directory.

Change-Id: I8d41840eec69eec1ef08527d8d71786612a3121d
Reviewed-on: https://go-review.googlesource.com/65152
Run-TryBot: Tom Bergan <tombergan@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tom Bergan <tombergan@google.com>
3 files changed