This allows easily adding additional information when uploading
existing files. Example usage:

benchsave -header <(echo goarch: arm; echo goos: linux) *.log

Change-Id: I46afafd39d8314d9c06c209f868b71cd5e58c63a
Reviewed-on: https://go-review.googlesource.com/35350Reviewed-by: Russ Cox <rsc@golang.org>