internal/export/idna: make API compatible with x/net/idna Also include a profile for displaying IDNs as recommended by UTS 46. Updates golang/go#17268 Change-Id: I33189fa8115e7891dbf21ba222ca28ce294437e2 Reviewed-on: https://go-review.googlesource.com/31274 Run-TryBot: Marcel van Lohuizen <mpvl@golang.org> Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>