GitLab

Additional elements in a DN can be added in via ExtraNames. This
option can also be used for sorting DN elements in a custom order.

Change-Id: Ie408d332de913dc2a33bdd86433be38abb7b55be
Reviewed-on: https://go-review.googlesource.com/2257

Reviewed-by: Adam Langley <agl@golang.org>