internal/export/idna: make API compatible with x/net/idna
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 <a22d082192f3ccf984644a4f482781f4c2d65180@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/text