benchstat: use template to generate html

The generated html changes in a few minor ways,
either unavoidable or not worth working around:

- Trailing <td></td> at end of row no longer removed.
- <td>~</td> instead of <td>~    </td>
- Literal + escaped to &#43;

Change-Id: I73a0d1f74ff4c3597dd96b1a4dac2d6a3d5cc462
Reviewed-on: https://go-review.googlesource.com/35938
Reviewed-by: Quentin Smith <quentin@golang.org>
8 files changed