benchstat: html table adjustments

Now that it's one table, move configurations in general case
to top row, not repeated once per metric.

Change-Id: I3c5c53da09b98a6d010ce51cdd7326ce5ebd635e
Reviewed-on: https://go-review.googlesource.com/35940
Reviewed-by: Quentin Smith <quentin@golang.org>
6 files changed