internal/ucd: added CommentHandler option

This is used parsing the idna table, which categorizes tests
in the comments.

Change-Id: I95ea73a6f9de1e15bf680f4538e03ab3c74954b2
Reviewed-on: https://go-review.googlesource.com/30551
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>