misc/sortac: add tool to sort the AUTHORS and CONTRIBUTORS files

I keep losing this utility, used as part of other tools to auto-update
the AUTHORS and CONTRIBUTORS files.  Check it in to the repo so I
don't lose it, and so others can use it as well.

Updates #12042

Change-Id: Ib5886b85799087aaaddcec4c81169e2726322c05
Reviewed-on: https://go-review.googlesource.com/16824


Reviewed-by: Russ Cox <rsc@golang.org>